# ¨U B S C H - K R A W ON B E R G L U N D - H I T ZM I R R O R S Y ME T R Y

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## Acknowledgments

So many people have somehow directly or indirectly influenced my viewpoint in mathematics. When I first sat down to write these acknowledgments, I realized how much of a social mathematician I am. I am grateful that there is such a community that I can talk to so many members of it. If your name got missed here, I am sorry. Thank you.

Firstly, I want to thank my advisor, Ron Donagi, who has supported me so much for such a long period of time. I am thankful so much for how much he not only helped me if I got stuck in research but how much he coached, mentored and taught me how to become a researcher. Looking back in time, I can see how much he has changed my outlook on mathematics and helped my viewpoint grow. Thank you, Ron, for being a role model, teacher, advisor, and friend.

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dissertation.

Thirdly, I would like to thank Charles Doran. He was the first external point of contact and support I had. Over the last year, we created a solid research bond. He has consistently been there to help me understand a new side to the narrative I am trying to present to the broader mathematical community and to help me grow as a member of the mathematical research community. His support and interest has been a motivator when I am worried I am working on something that no one cares about.

I thank all the senior faculty at Penn. They have been so supportive of my mathematical freedom and maintaining an environment where mathematics can be done. In particular, I would like to thank all the faculty that have been willing to drop everything and have a spontaneous conversation about things and have helped me grow a better perspective of the whole realm of mathematics: Florian Pop, David Harbater, Stephen Shatz, Antonella Grassi, Robert Ghrist, and Andreea Nicoara.

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half a decade. Paula, you have been there to give me a hug throughout the good and bad news. I truly am one of your children.

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Dyckerhoff, and the list continues.

Outside of Penn, I have had numerous interactions with other students and faculty that I have found helpful mathematically. A special thank you to the other members of the BIRS Focussed Research Group on Effective Computations in Arithmetic Mirror Symmetry: Ursula Whitcher, Adriana Salerno, Xenia de la Ossa, Stephen Sperber, Andrew Harder, Andreas Malmendier, and Charles Doran. Thank you also to David Favero, Dave Jensen, Dave Swinarski, and Colin Diemer for all their explanations and comments on GIT. Thank you to Alan Thompson, Nathan Priddis, and Alessandra Sarti for your conversations on K3 surfaces which motivates Chapter Four. Thank you George Schaeffer for telling me what factor-ization theory does which yields an example in Chapter Five. Thank you to Ted Shifrin, Ryan Keast, Robert Klinzmann, and George Schaeffer for being my personal support from a mathematical level throughout these last five years.

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Schaeffer, Sean Masterton, Phil Cochetti, Roger Poulard, Robert Odri, David Muir, Jared Weinstein, Joel Morrow, Grant Mundell, Robert Blue, Ashley Allen, Chris Goodman, and Lindsey Scott Goodman. Y’all’re awesome. I love y’all.

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ABSTRACT

ON BERGLUND-H ¨UBSCH-KRAWITZ MIRROR SYMMETRY

Tyler L. Kelly Ron Y. Donagi

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## Contents

1 Introduction 1

2 Background 9

2.1 What is Mirror Symmetry? . . . 9

2.2 Mirror Constructions . . . 11

2.2.1 Greene-Plesser Mirrors . . . 12

2.2.2 Batyrev-Borisov Duality . . . 14

2.3 Berglund-H¨ubsch-Krawitz Mirror Symmetry . . . 15

2.3.1 Delsarte Hypersurfaces in Weighted Projective Space . . . . 15

2.3.2 Group of Diagonal Automorphisms . . . 17

2.3.3 The BHK Mirror . . . 20

2.3.4 Classical Mirror Symmetry for BHK Mirrors . . . 22

2.3.5 Comparison between Mirror Constructions . . . 24

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3.1.1 Reinterpretation of the Dual Group . . . 29

3.1.2 Birational Geometry of BHK Mirrors . . . 30

3.2 Multiple Mirrors . . . 32

3.3 Example: BHK Mirror 3-folds, Shioda Maps, Chen-Ruan Hodge Numbers . . . 36

3.3.1 BHK Mirrors . . . 37

3.3.2 Shioda Maps . . . 38

3.3.3 Chen-Ruan Hodge Numbers . . . 40

4 On the Picard Ranks of Surfaces of BHK-Type 45 4.1 Introduction . . . 45

4.2 Surfaces of BHK-Type . . . 49

4.2.1 Weighted Delsarte Surfaces . . . 49

4.2.2 Symplectic Group Actions . . . 51

4.2.3 The Berglund-H¨ubsch-Krawitz Mirror . . . 54

4.3 Picard Ranks of Surfaces of BHK-Type . . . 54

4.3.1 Hodge Theory on Fermat Surfaces . . . 55

4.3.2 Picard Ranks of K3 Surfaces of BHK Type . . . 56

4.4 An Example . . . 60

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5.2 Dual Gorenstein Cones and the Unification Framework . . . 69

5.2.1 Notation and the Geometry of Dual Convex Cones . . . 69

5.2.2 Gorenstein Cones and their Splittings . . . 73

5.2.3 Cone Closures and Vector Bundles . . . 75

5.3 Batyrev Duality . . . 81

5.3.1 Total spaces of line bundles . . . 81

5.3.2 The case of normal fans to reflexive polytopes . . . 84

5.4 Batyrev-Borisov Duality . . . 86

5.4.1 Toric Vector Bundles and Gorenstein Cones . . . 86

5.4.2 Nef Partitions and Batyrev-Borisov Duality . . . 90

5.5 Berglund-H¨ubsch Duality . . . 92

5.6 Examples of higher index Gorenstein cones with respect to Berglund-H¨ubsch Symmetry . . . 96

5.6.1 Example One . . . 96

5.6.2 Example Two . . . 99

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## Introduction

The mirror symmetry conjecture predicts that for a Calabi-Yau variety, M, there exists another Calabi-Yau variety, W, so that various geometric and physical data is exchanged between M and W. A classical relationship found between so-called mirror pairs is that on the level of cohomology

Hp,q(M,C)∼=HN−p,q(W,C),

provided that both Calabi-Yau varietiesM and W areN-dimensional. Suppose FA is a polynomial,

FA= n

X

i=0 n

Y

j=0

xaij

j , (1.0.1)

whereaij ∈N, so that there exist positive integersqj anddso that Pjaijqj =dfor all i ( i.e., FA is quasihomogeneous). The polynomial FA cuts out a hypersurface

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equivalent to P

iqi =d and see Subsection 2.3.1 for details about the quasismooth condition). Greene and Plesser proposed a mirror to XA when the polynomial FA was Fermat . Their proposed mirror for the hypersurface XA was a quotient of

XA by all its phase symmetries of XA leaving the cohomology Hn,0(XA) invariant. The problem was that their proposal does not work well for the case when XAwas not a Fermat hypersurface. Berglund and H¨ubsch proposed that the mirror of the hypersurface XA should relate to a hypersurface XAT cut out by

FAT =

n

X

i=0 n

Y

j=0

xaji

j . (1.0.2)

The hypersurface XAT sits inside a different weighted-projective 4-space, denoted WPn(r

0, . . . , rn). Berglund and H¨ubsch proposed that the mirror ofXA should be a quotient of this new hypersurface XAT by a suitable subgroup P of the phase

symmetries. In several examples, they showed that XA and XAT/P satisfy the

classical mirror symmetry relation in that

hp,q(XA,C) =hn−1−p,q(XAT/P,C).

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SL(FA) of phase symmetries ofFAleavingHn,0(XA) invariant. Define the subgroup

JFA of SL(FA) to be the group consisting of the phase symmetries induced by the

C∗ action on weighted-projective space (so that all elements of JFA act trivially on

the weighted-projective space). Take the group G to be some subgroup of SL(FA) containing JFA, i.e., JFA ⊆G⊆SL(FA). We obtain a Calabi-Yau orbifold ZA,G := XA/G˜ where ˜G := G/JFA. Consider the analogous groups SL(FAT) and JFAT for

the polynomial FAT. Krawitz defined the dual group GT relative to G so that JFAT ⊆GT ⊆SL(FAT). For precise definitions of these groups, we direct the reader

to Section 2.3. Take the quotient ˜GT :=GT/J

FAT. The Berglund-H¨ubsch-Krawitz mirror to the orbifold ZA,G is the orbifold ZAT,GT := XAT/G˜T. Chiodo and Ruan

proved the classical mirror symmetry statement for the mirror pairZA,GandZAT,GT

is satisfied on the level of Chen-Ruan cohomology :

HCRp,q(ZA,G,C)∼=Hn

−1−p,q

CR (ZAT,GT,C). (1.0.3)

This dissertation is a suite of projects discussing Berglund-H¨ubsch-Krawitz mir-rors.

Chapter 2 provides the background about the field, starting with a short ex-position on the field of mirror symmetry and mirror constructions, then moving to describing Berglund-H¨ubsch-Krawitz (BHK) Mirror Symmetry in detail. The chap-ter finishes with a discussion comparing BHK mirror mirrors with Batyrev-Borisov mirrors.

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mirror symmetry is that it proposes possibly distinct mirrors of isolated points of the family in the Calabi-Yau moduli space–not mirrors of families like the work of Batyrev and Borisov. These BHK mirrors of the isolated points may not live in the same family. Suppose one starts with two quasihomogeneous potentials FAand FA0

FA = n

X

i=0 n

Y

j=0

xaij

j ; FA0 =

n

X

i=0 n

Y

j=0

xa

0

ij

j . (1.0.4)

Assume that there exist positive integers qi, qi0 so that

XA =Z(FA)⊆WPn(q0, . . . , qn);

XA0 =Z(FA0)⊆WPn(q0

0, . . . , q

0

n)

(1.0.5)

and that XA and XA0 are Calabi-Yau. Take G and G0 to be subgroups of the

group of phase symmetries that leave the respective cohomologies Hn,0(X

A,C) and

Hn,0(X

A0,C) to be invariant. We obtain two Calabi-Yau orbifolds ZA,G and ZA0,G0.

One can find examples ZA,GandZA0,G0 in the same family where their BHK mirrors

ZAT,GT and Z(A0)T,(G0)T will be quotients of hypersurfaces in different

weighted-projective spaces. See Section 3.3 for an explicit example.

Since the mirrors proposed by BHK and Batyrev-Borisov mirror symmetry are different, we ask the question of how we can relate them. Iritani suggested to look at the birational geometry of the mirrors ZAT,GT and Z(A0)T,(G0)T. In this paper, we

prove the following theorem.

Theorem 1.0.1. LetZA,GandZA0,G0 be Calabi-Yau orbifolds as above. If the groups

G andG0 are equal, then the BHK mirrors ZAT,GT andZ(A0)T,(G0)T of these orbifolds

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In dimension 3, one then has that the BHK mirrors ZAT,GT and Z(A0)T,(G0)T are

derived equivalent, due to a theorem of Bridgeland. A key technical idea for proving Theorem 1.0.1 is using Shioda maps. Originally, Shioda used these maps to compute Picard numbers of Delsarte surfaces in . These maps entered the multiple mirror literature in  where they were generalized and then used to investigate Picard-Fuchs equations of different pencils of quintics in P4. The Shioda maps were then further generalized to look at GKZ hypergeometric systems for certain families of Calabi-Yau varieties in weighted-projective space in . This chapter provides a concrete description of how Shioda maps relate to BHK mirror symmetry than the previous two papers, and explains the groups used in the theorems of  and  in the context of BHK mirrors. The chapter ends with an explicit example that shows explicitly a full example where the Calabi-Yau orbifolds ZAT,GT andZ(A0)T,(G0)T are

birational and the Chen-Ruan cohomology decomposes in different ways.

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by so-called mirror lattices. In particular, these lattices embed into the subgroup of the Picard groups of the BHK mirrors that are invariant under the non-symplectic automorphism induced on the K3 surface due to the fact of it being a prime cover of P2. The fact that it is a prime cover of

P2 requires that the polynomial be of the form

FA:=xa000 + n

X

i=1 n

Y

j=1

xaij

j , (1.0.6)

where a00 is a prime number.

In this paper, we drop this hypothesis and investigate the Picard ranks. The key tools that we use are Shioda maps and information about the middle cohomology of Fermat varieties. We use a Shioda map to relate each surface of BHK-type birationally to a quotient of a higher degree Fermat hypersurface in projective space by a finite group H. We then describe the H-invariant part of the transcendental lattice of the Fermat hypersurface, which gives us the rank of the transcendental lattice of the surface of BHK type, i.e., the Lefschetz number. Recall that for an algebraic surface X, the Lefschetz number λ(X) is defined to be

λ(X) := b2(X)−ρ(X). (1.0.7)

Take BHK mirrors surfaces ZA,G and ZAT,GT as above over a field of

character-istic p. Take d to be a positive integer so that the matrix dA−1 has only integer entries. Let Id(p) be the subset of symmetries on a degree d Fermat hypersurface

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Q, Tn(Xd) (See Section 4.3.1 for an explicit description of Id(p) that is very com-putable). We remark that the subset Id(p) depends on the characteristic p of the field k. We then describe the rank of the Picard group. In particular, we prove the following theorem:

Theorem 1.0.2. The Lefschetz numbers of the BHK mirrorsZA,G and ZAT,GT are: λ(ZA,G) = #(Id(p)∩GT) and

λ(ZAT,GT) = #(Id(p)∩G).

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The surprise is that the dual group GT associated to the BHK Mirror ZAT,GT

actually plays a role in the computation of the Lefschetz number of the original K3 orbifold ZA,G. We can see explicitly a nice correspondence between the mirrors in this fashion. This theorem has the following corollary:

Corollary 1.0.3. The Picard ranks of the BHK mirrors ZA,G and ZAT,GT are: ρ(ZA,G) = 22−#(Id(p)∩GT) and

ρ(ZAT,GT) = 22−#(Id(p)∩G).

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An added quick corollary is a lower bound on the Picard number of a BHK mirror is by the order of dual group GT. Also, a great benefit to this is that the Picard number of each BHK mirror surface is now computable, once one chooses over which field one works. We end the chapter with an example where we compute the Picard ranks of a BHK mirror pair over various fields and see that there are examples of primes p where either one, both or neither of the surfaces ZA,G and

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## Background

### What is Mirror Symmetry?

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MK(M)

Mc−x(M)

M

Mc−x(W)

MK(W)

W

Figure 2.1: (2,2) Mirror Symmetry for M and its mirror W

so that Mc-x(Y) corresponds toMK(X) and MK(Y) corresponds to Mc-x(X): On the level of cohomology, for two Calabi-Yau threefolds M and W, this pic-ture means that their Hodge diamonds ‘flip’, i.e., there is an isomorphism among cohomology groups

Hp(M,ΩqM)∼=H3−p(W,ΩqW),

where ΩM and ΩW are the cotangent bundles of M and W, respectively. Gen-eralizing this framework, mirror symmetry predicts that, given an n-dimensional Calabi-Yau varieties M, there exists anotherW so that their Hodge diamonds ‘flip’ and one has:

Hp(M,ΩqM)∼=Hn−p(W,ΩqW).

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In his 1994 ICM plenary lecture, Kontsevich introduced the notion of Homologi-cal Mirror Symmetry, a conjectural formulation that mirror symmetry categoriHomologi-cally exchanges the symplectic data of M with the complex geometry of W. In , Kontevich proposed the following equivalence. Take (M, ω) to be a 2n-dimensional symplectic manifold with vanishing first Chern class c1(M) = 0 andW to be a dual

n-dimensional complex algebraic manifold.

Conjecture 2.1.1 (Kontevich’s Homological Mirror Conjecture as stated in ). The derived category constructed from the Fukaya categoryF(V) (or a suitably enlarged one) is equivalent to the derived category of coherent sheaves on a complex algebraic variety W.

Over the past twenty years, there has been very promising work on proving this conjecture and many refinements of this conjecture have been stated. For example, Sheridan recently proved Homological Mirror Symmetry for Calabi-Yau hypersurfaces in projective space .

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### Greene-Plesser Mirrors

Greene and Plesser introduced the idea that one can find a mirror to certain Calabi-Yau hypersurfaces in weighted-projective space by quotienting by certain discrete symmetries of certain type and then taking the crepant resolution of the singu-larities. In their work , they look at the example of the Fermat hypersurface

X5 = {x51 +x52 +x35 +x54+x55 = 0} ⊆ P4. There are many choices of a group G by which one can quotient X5 that will create a Calabi-Yau orbifold X5/G, i.e., when one quotients the hypersurface X5 by the groupGand then take the minimal resolution X^5/G99K(X5/G) so that X^5/Gis still a Calabi-Yau manifold.

Let us comment on the possible groupsG. LetHbe the group of automorphisms found by multiplying by the roots of unity on each coordinate while keeping the nonvanishing holomorphic 3-form invariant, i.e.,

H =

(

(e2πia1/5, . . . , e2πia5/5)

ai ∈Z, 5

X

j=1

ai ∈5Z

)

.

This does not act nontrivially on the hypersurface. The subgroup J that acts trivially is generated by the element (e2πi/5, . . . , e2πi/5). QuotientH byJ to make a group ¯H =H/J. For notation, we will denote the element (e2πia1/5, . . . , e2πia5/5) by

[a1, a2, a3, a4, a5]. One can look at all the possible quotients of the hypersurface X5 by subgroups of ¯H. Greene and Plesser compute the cohomology values h1,1 and

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Generators of ¯G h2,1 h1,1

0 101 1

[0,0,0,1,4] 49 5 [0,1,2,3,4] 21 1 [0,1,1,4,4], [0,1,2,3,4] 21 17

[0,1,1,4,4] 17 21 [0,1,3,1,0], [0,1,1,0,3] 1 21 [0,1,4,0,0], [0,3,0,1,1] 5 49 [0,1,2,3,4], [0,1,1,4,4], [0,0,0,1,4] 1 101

Table 2.1: Quotients of X5

Hodge numbers are exchanged, providing evidence of a mirror pair. This phenomenon was made rigorous for polynomials of the form

FA= n

X

i=0 n

Y

j=0

xaij

j

where A = (aij)ni,j=0 is an (n + 1) by (n + 1) matrix satisfying certain proper-ties. Quotients of the zero locus of FA in weighted projective space are related to quotients of the zero locus of FAT. This mirror duality was proposed in 1992 by

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made rigorous by Krawitz in . For an explicit description of Berglund-H¨ ubsch-Krawitz mirror symmetry, we direct the reader to Section 2.3.

### Batyrev-Borisov Duality

Next, let us explain the toric approach of Batyrev and Borisov. Let M and N be dual lattices. One starts with a Gorenstein Fano toric variety P∆ associated to a lattice polytope ∆⊂MR:=M⊗R, i.e., all the vertices of the polytope ∆ are in the lattice M. The polytope associated to a Gorenstein Fano toric variety is reflexive. Recall the definition of reflexive is that the dual polytope

∆∗ :={n ∈N|hm, ni ≥ −1 for all m∈∆}

is a lattice polytope in N. Note that this requires that the polytopes ∆ and ∆∗ to both have exactly one lattice point in their interiors. Moreover, note that if the polytope ∆ is reflexive, then the dual polytope ∆∗ is reflexive as well. This implies that the toric variety P∆∗ associated to the polytope ∆∗ is One can construct

fami-lies F(∆) andF(∆∗) of hypersurfaces inP∆ and P∆∗ that satisfy certain regularity

conditions that imply that the singularities of the hypersurface are induced by the ambient toric varieties P∆ and P∆∗. In , Batyrev proposes the combinatorial

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Hodge numbers h2,1 and h1,1 were exchanged amongst the two hypersurfaces. This proposal was generalized by Batyrev and Borisov to explain the case of a regular Calabi-Yau complete intersections in Gorenstein Fano toric varieties  and .

We now give an exposition about an alternate mirror construction to that of Batyrev and Borisov, the Berglund-H¨ubsch-Krawitz mirror construction, for certain hypersurfaces in quotients of weighted projective space.

### Delsarte Hypersurfaces in Weighted Projective Space

We start with a matrix A with nonnegative integer entries (aij)ni,j=0. Define a polynomial

FA= n

X

i=0 n

Y

j=0

xaij

j and impose the following conditions:

1. the matrix A is invertible;

2. the polynomialFAis quasihomogeneous, i.e., there exist positive integersqj, d so that

n

X

j=0

aijqj =d,

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3. the polynomial FA is a non-degenerate potential away from the origin, i.e., we are assuming that, when viewing FA as a polynomial in Cn+1, Z(FA) has exactly one singular point (at the origin).

Remark 2.3.1. These conditions are restrictive. By Theorem 1 of , there is a classification of such polynomials. That is, FA can be written as a sum of invertible potentials, each of which must be of one of the three so-called atomic types:

WFermat:=xa,

Wloop:=xa11x2 +xa22x3+. . .+x am−1

m−1xm+xammx1, and

Wchain :=xa11x2 +xa22x3+. . . xamm−−11xm+xamm.

(2.3.1)

Using Condition (1), we define the matrix B = dA−1, where d is a positive integer so that all the entries of B are integers (note that d is not necessarily the smallest such d). Take e:= (1, . . . ,1)T

Rn and

q :=Be, i.e., qi =

X

j

bij.

Then the polynomial FA defines a zero locus XA = Z(FA) ⊆ WPn(q0, . . . , qn). Indeed, with these weights, the polynomialFAis quasihomogeneous: each monomial in FA has degree

Pn

j=0aijqj =d, as Aq =ABe= de. Condition (2) above is used to ensure that each integer qi is positive.

Assume further that P

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Sing(WPn(q

0, . . . , qn))∩ XA. Condition (3) above implies that our hypersurface

XA is quasi-smooth. We remark that Condition (1) is used once again when we introduce the BHK mirror in Section 2.2: it ensures that the matrix AT is a matrix of exponents of a polynomial with n+ 1 monomials and n+ 1 variables that also satisfies Conditions (1), (2), and (3).

### Group of Diagonal Automorphisms

Let us discuss the groups of symmetries of the Calabi-Yau variety XFA. Firstly,

consider the scaling automorphisms of the set Cn+1 \ {0} when n ≥ 2. There is a subgroup, (C∗)n+1, of the automorphisms of

Cn+1 \ {0}. Explicitly, an element (λ0, . . . , λn)∈(C∗)n+1 acts on any elementx= (x0, . . . , xn)∈Cn+1\ {0} by:

(λ0, . . . , λn)×(x0, . . . , xn)7−→(λ0x0, . . . , λnxn).

We view the weighted projective n-space WPn(q0, . . . , qn) as a quotient of Cn+1\

{0} by a subgroup C∗ ⊂ (C∗)n+1 consisting of the elements that can be written (λq0/d, . . . , λqn/d) for someλ

C∗.

Moreover, there is a subgroup of (C∗)n+1, denoted Aut(FA), which can be defined as

Aut(FA) :=

(λ0, . . . , λn)∈(C∗)n+1 | FA(λ0x0, . . . , λnxn) =FA(x0, . . . , xn)

for all (x0, . . . , xn) .

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the group of scaling symmetries. Note that for (λ0, . . . , λn) to be an element of Aut(FA), each monomial

Qn

j=0x aij

j in the polynomial FA must be invariant under the action of (λ0, . . . , λn).

Using the classification of Kreuzer and Skarke (see Remark 2.3.1), we can see that for any polynomial of one of the atomic types that each λi must have modulus 1. If the polynomial FA is of Fermat-type, then λaxa = xa hence λa = 1. If FA is of loop-type, then λai

i λi+1 = 1 for all i < am, hence λi+1 = λ−i ai. Moreover,

λam

m λ1 = 1 hence λ1 = λ−mam = λ amam−1

m−1 = · · · = λ (−1)ma

1···am

1 . If |λ| 6= 1 then (−1)ma

1. . . am = 1. This would require m to be even and ai to be 1 for all i. However, then the degree of the polynomial,d, must beq1+q2; howeverd=Pni=0qi,

n ≥ 2, and qi >0, hence a contradiction is reached. Lastly, if FA is of chain-type,

λam

m xamm =xamm, hence |λm|am = 1. This implies that |λ am−1

m−1λm| =|λ am−1

m−1|= 1, and so on, hence |λi| = 1. Any polynomial that is a combination of such types has an analogous argument.

Since each λi can be written as eiθi, for some θi ∈ R, we can then see that (λ0, . . . , λn) ∈ Aut(FA) if and only if we have that Qnj=0eiaijθj = 1 for all i. The map (λ0, . . . , λn)7→(21πilog(λ0), . . . ,21πilog(λn)) induces an isomorphism

Aut(FA)∼=

              

(z0, . . . , zn)∈(R/Z)n

A         z0 .. . zn        

Zn+1

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We then observe that we can describe Aut(FA) as being generated by the elements

ρi = (e2πib0i/d, . . . , e2πbni/d)∈(C∗)n+1.

Moreover, there is a characterization by Artebani, Boissi`ere, and Sarti of the group Aut(FA) (Proposition 2 of ):

Proposition 2.3.2. Aut(FA) is a finite abelian group of order |detA|. If we think of FA as a sum of atomic types, FA1(x0, . . . , xi1) +. . .+FAk(xik−1+1, . . . , xn), then

we may characterize the elements of Aut(FA) as being the product of the k groups Aut(FAi). The groups Aut(FAi) are determined based on the atomic types:

1. For a summand of Fermat type WFermat = xa, the group Aut(WFermat) is

isomorphic to Z/aZ and generated by ϕ=e2πi/a C∗.

2. For a summand of loop type Wloop =xa11x2+xa22x3+. . .+xamm−−11xm+xammx1,

the group Aut(Wloop) is isomorphic to Z/ΓZ where Γ = a1· · ·am + (−1)m+1

and generated by (ϕ1, . . . , ϕm)∈(C∗)m, where

ϕ1 :=e2πi(−1)

m/Γ

, and ϕi :=e2πi(−1)

m+1−ia

1···ai−1/Γ, i2.

3. For a summand of chain type,Wchain =xa11x2+xa22x3+. . . x am−1

m−1xm+xamm, the

groupAut(Wchain)is isomorphic toZ/(a1· · ·am)Z, and generated by(ϕ1, . . . , ϕm)∈ (C∗)m, where

ϕi =e2πi(−1)

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Note that the subgroups C∗ and Aut(FA) have nontrivial intersection. Let

JFA := Aut(FA)∩C

. The group J

FA is generated by (e

2πiq0/d, . . . , e2πiqn/d), which

is clearly in Aut(FA) because

Pn

j=0aijqj =dand the alternate description provided by the isomorphism above in Equation 2.3.3 (moreover, (e2πiq0/d, . . . , e2πiqn/d) = Qn

i=0ρi ∈Aut(FA)).

We now introduce the group

SL(FA) :=

(

(λ0, . . . , λn)∈Aut(FA)

n Y j=0

λj = 1

)

.

The groupJFA is a subgroup ofSL(FA) as a generator ofJFAis the element (e

2πiqj/d)

j and Q

je

2πiqj/d =e2dπi P

jqj = 1. Fix a group Gso that J

FA ⊆G⊆SL(FA) and put

˜

G:=G/JFA. To help summarize, we have the following diagram of groups:

JFA _

 //J

FA _

 //J

FA _

 //

C∗ _

G  //

SL(FA)  //Aut(FA)  //(C∗)n+1

˜

G:=G/JFA (C

)n+1/ C∗ Consider the Calabi-Yau orbifold, ZA,G := XFA/G˜ ⊂ WP

n(q

0, . . . , qn)/G˜. We now will describe the Berglund-H¨ubsch-Krawitz mirror to it.

### The BHK Mirror

In this section, we construct the BHK mirror to the Calabi-Yau orbifoldZA,Gdefined above. Take the polynomial

FAT =

n X i=0 n Y j=0

Xaji

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It is quasihomogeneous because there exist positive integers ri :=Pjbji so that

FAT(λr0X0, . . . , λrnXn) = λdFAT(X0, . . . , Xn). (2.3.5)

Note that the polynomial FAT cuts out a well-defined Calabi-Yau hypersurface XAT ⊆WPn(r0, . . . , rn). Define the diagonal automorphism group, Aut(FAT),

anal-ogously to Aut(FA). By the analogous isomorphism to that in Equation 2.3.3, the group Aut(FAT) is generated by ρTi := diag(e2πibij/d)nj=0∈(C∗)n+1. Define the dual

group GT relative to Gto be

GT :=

( n Y

i=0 (ρTi )si

si ∈Z, where n

Y

i=0

xsi

i is G-invariant

)

⊆Aut(FAT). (2.3.6)

Lemma 2.3.3. If the group G is a subgroup of SL(FA), then the dual group GT

contains the group JFAT.

Proof. It is sufficient to show that the element Qn

j=0ρ T

j is in the dual group GT. This is equivalent to Qn

j=0xj to be G-invariant. Any element (λ0, . . . , λn) of Gacts on the monomial Qn

j=0xj by

Qn

j=0λj = 1 (as G⊆SL(FA)).

Lemma 2.3.4. If the group G contains JFA, then the dual group G

T is contained

in SL(FAT).

The proof of this lemma is analogous to the lemma above. As the dual group

GT sits between J

FAT and SL(FAT), define the group ˜GT := GT/JF

AT. We have a

well-defined Calabi-Yau orbifold ZAT,GT := XAT/G˜T ⊂ WPn(r0, . . . , rn)/G˜T. The

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### Classical Mirror Symmetry for BHK Mirrors

In this section, we summarize some results of Chiodo and Ruan for BHK mirrors. This section is based on Section 3.2 of . We recommend the exposition there. Recall that we can view the weighted projective n-spaceWPn(q

0, . . . , qn) as a stack

Cn+1\ {0}/C∗ (2.3.7)

where a group element λ of the torus C∗ acts by

λ·(x0, . . . , xn) = (λq0x0, . . . , λqnxn). (2.3.8)

The quotient stack WPn(q0, . . . , qn)/G˜ is equivalent to the stack

Cn+1\ {0}/GC∗ (2.3.9)

so we can view the Calabi-Yau orbifold ZA,G as the (smooth) Deligne-Mumford stack

[ZA,G] :=

x∈Cn+1\ {0}|F

A(x) = 0 /GC∗

Cn+1\ {0}/GC∗. (2.3.10)

We now review the Chen-Ruan orbifold cohomology for such a stack. Intuitively speaking, it consists of a direct sum over all elements of GC∗ of G-invariant coho-mology of the fixed loci of each element.

Ifγ is an element of GC∗, take the fixed loci

Cnγ+1 : ={x∈C n+1\ {

0}|γ·x=x}; and

XAγ : ={FA|Cn+1

γ = 0} ⊂C

n+1 γ .

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Fix a point x ∈ XAγ. The action of γ on the tangent space Tx({FA = 0}) can be written as a diagonal matrix (when written with respect to a certain basis), Λγ = diag(e2πia

γ

1, . . . , e2πia

γ

n), for some real numbersaγ

i ∈[0,1). We then define the

age shift of γ,

a(γ) := 1

2πilog(det Λγ) =

n

X

j=1

j. (2.3.12)

We now may define the bigraded Chen-Ruan orbifold cohomology as a direct sum of twisted sector ordinary cohomology groups:

HCRp,q([ZA,G],C) =

M

γ∈GC∗

Hp−a(γ),q−a(γ)(XAγ/GC∗,C). (2.3.13)

The degree d Chen-Ruan orbifold cohomology is defined to be the direct sum

HCRd ([ZA,G],C) =

M

p+q=d

HCRp,q([ZA,G],C). (2.3.14)

Continue to assume that the groupGcontainsJFA and is a subgroup ofSL(FA)

and the hypersurface XA is Calabi-Yau. Chiodo and Ruan prove:

Theorem 2.3.5 (Theorem 2 of ). Given the Calabi-Yau orbifold ZA,G and its

BHK mirror ZAT,GT as above, one has the standard relationship between the Hodge

diamonds of mirror pairs on the level of the Chen-Ruan cohomology of the orbifolds:

HCRp,q([ZA,G],C)∼=Hn

−1−p,q

CR ([ZAT,GT],C).

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Corollary 2.3.6 (Corollary 4 of ). Suppose both Calabi-Yau orbifolds ZA,G and

ZAT,GT admit smooth crepant resolutions M and W respectively, then we have the

equality

hp,q(M,C) = hn−1−p,q(W,C),

where hp,q is the ordinary (p, q) Hodge number.

### Comparison between Mirror Constructions

In this subsection we will comment on the different mirrors that BHK and Batyrev-Borisov mirror symmetry propose for hypersurfaces in quotients of weighted pro-jective space. In Sections 5.4 and 5.5 of , Batyrev pays close attention to the case where the reflexive polytopes ∆ and ∆∗ are simplices, which corresponds to the toric varieties P∆and P∆∗ being quotients of weighted projective spaces. Let ∆

be a polytope associated to a weighted projective space with weights qi. Batyrev proves that the family of Calabi-Yau hypersurfaces F(∆) consists of deformations of Fermat-type hypersurfaces. He also proves that the toric variety ∆∗ will be a quotient of the weighted projective space with weights qi. This is in contrast to the BHK construction where the mirrors can be hypersurfaces in quotients of different weighted projective spaces. See for example in Table 2.2 that is taken from  that computes the BHK mirrors to quintics in projective 4-space.

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Batyrev-Borisov mirror duality. When one has multiple mirrors for a Calabi-Yau manifold, one can ask a variety of questions. In his 1994 paper outlining homological mirror symmetry, Kontsevich explains that “the B-models on birationally equiva-lent Calabi-Yau manifolds W and W0 are believed to be isomorphic.... we expect that the derived categories of coherent sheaves on W and on W0 are equivalent.

Kontsevich’s viewpoint provides the following question:

Question 2.3.7. IfFAandFA0 are polynomials as above andGandG0 are groups so

that JFA ⊂G⊂SL(FA) andJFA0 ⊂G

0 SL(F

A0) so that the Calabi-Yau orbifolds

ZA,G and ZA0,G0 are in the same toric variety, do we have a birational or derived

equivalence between ZAT,GT and Z(A0)T,G0T?

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QuinticZA,G⊆P4 BHK MirrorZAT,GT

1 {x5

0+x51+x52+x53+x54= 0} ⊂P4

{y5 0+y

5 1+y

5 2+y

5 3+y

5

4= 0} ⊂P 4

(Z5)3: [4,1,0,0,0],[4,0,1,0,0],[4,0,0,1,0]

2 {x4

0x1+x51+x 5 2+x

5 3+x

5

4= 0} ⊂P

4 {y

4

0+y0y51+y25+y35+y54= 0} ⊂WP4(5,3,4,4,4)

(Z5)2: [0,0,4,1,0],[0,0,4,0,1]

3 {x4

0x1+x41x2+x25+x53+x54= 0} ⊂P4

{y4

0+y0y41+y1y52+y 5 3+y

5

4= 0} ⊂WP

4(20,15,13,16,16)

Z5: [0,0,0,4,1]

4 {x4

0x1+x41x2+x42x3+x53+x 5

4= 0} ⊂P

4 {y4

0+y0y41+y1y42+y2y53+y 5

4= 0} ⊂WP

4(80,60,65,51,64)

5 {x4

0x1+x41x2+x42x3+x43x4+x54= 0} ⊂P4 {y40+y0y41+y1y42+y2y34+y3y54= 0} ⊂WP4(64,48,52,51,41)

6 {x4

0x1+x51+x 4

2x3+x53+x 5

4= 0} ⊂P

4 {y4

0+y0y15+y 4

2+y2y35+y 5

4= 0} ⊂WP

4(5,3,5,3,4)

7 {x4

0x1+x41x2+x52+x 4

3x4+x54= 0} ⊂P

4 {y4

0+y0y41+y1y52+y 4

3+y3y55= 0} ⊂WP

4(20,15,13,20,12)

8 {x5 0+x

5 1+x

5 2+x

4

3x4+x44x3= 0} ⊂P4 {y

5 0+y

5 1+y

5 2+y

4

3y4+y3y44= 0} ⊂P 4

(Z5)2×Z3: [1,4,0,0,0],[0,4,1,0,0],[0,0,0,2,1]

9 {x5 0+x

4

1x2+x52+x 4

3x4+x44x3= 0} ⊂P4 {y50+y 4

1+y1y25+y 4

3y4+y3y44= 0} ⊂WP

4(20,15,13,16,16)

10 {x4

0x1+x41x2+x52+x 4

3x4+x44x3= 0} ⊂P4 y4

0+y0y41+y1y25+y43y4+y3y44= 0} ⊂WP4(20,15,13,16,16)

Z15: [0,0,3,11,1]

11 {x5

0+x41x2+x42x1+x43x4+x44x3= 0} ⊂P4 {y

5 0+y

4

1y2+y1y42+y 4

3y4+y3y44= 0} ⊂P 4

Z3: [0,2,1,2,1]

12 {x5 0+x

5 1+x

4

2x3+x43x4+x44x2= 0} ⊂P4 {y

5 0+y

5 1+y

4

2y4+y2y34+y3y44= 0} ⊂P 4

Z5×Z13: [1,4,0,0,0],[0,0,3,9,1]

13 {x4

0x1+x51+x 4

2x3+x43x4+x44x2= 0} ⊂P4 {y

4

0+y0y15+y24y4+y2y34+y3y44= 0} ⊂WP4(5,3,4,4,4)

Z65: [0,52,16,61,1]

14 {x4

0x1+x41x0+x42x3+x43x4+x44x2= 0} ⊂P4 {y

4

0y1+y0y41+y 4

2y4+y2y34+y3y44= 0} ⊂P 4

Z3×Z13: [1,2,0,0,0],[0,0,3,9,1]

15 {x5 0+x

4

1x2+x42x3+x34x4+x44x1= 0} ⊂P4 {y

5

0+y41y4+y1y24+y2y43+y44y3= 0}

Z51: [0,38,16,47,1]

16 {x4

0x1+x41x2+x42x3+x43x4+x44x0= 0} ⊂P4 {y

4

0y4+y0y41+y1y24+y2y43+y3y44= 0} ⊂P 4

Z41: [10,18,16,37,1]

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## Mirrors

### Shioda Maps

We now introduce the Shioda map and relate it to BHK mirrors. Recall the hy-persurfaces XA and XAT as above. Define the matrix B to be dA−1 where d is a

positive integer so thatB has only integer entries. The Shioda maps are the rational maps

φB :Pn99KWPn(q0, . . . , qn), and

φBT :Pn 99KWPn(r0, . . . , rn),

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where

(y0 :. . .:yn) φB

7→(x0 :. . .:xn), xj = n

Y

k=0

ybjk

k , and

(y0 :. . .:yn) φBT

7→ (z0 :. . .:zn), zj = n

Y

k=0

ybkj

k .

(3.1.2)

Consider the polynomial

FdI := n

X

i=0

yid (3.1.3)

and the Fermat hypersurface cut out by it, XdI := Z(FdI) ⊂ Pn. Note that the Shioda maps above restrict to rational mapsXdI

φB

99KXA andXdI

φB

99KXAT,

respec-tively, allowing us to obtain the diagram:

XdI φB

|

|

zz

zz φBT

" " F F F F

XA XAT

(3.1.4)

We now reinterpret the groupsGandGT in the context of the Shioda map. Any element of Aut(FdI) is of the form g = (e2πihj/d)j, for some integers hj. When we push forward the action of g viaφB, we obtain the diagonal automorphism

(φB)∗(g) := (e 2πi

d

Pn

j=0bijhj)

i ∈Aut(FA). (3.1.5)

The element (φB)∗(g) is a generic element of Aut(FA), namely Qnj=0ρ

hj

j . We turn our attention to describing the dual group GT toG. If we push the element gT := (e2πisi/d)

i ∈Aut(FdI) down via the map φBT, then we get the action

(φBT)(gT) = (e

2πi d

Pn

i=0sibij)j =

n

Y

i=1

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In other words, we have (surjective) group homomorphisms

(φB)∗ : Aut(FdI)→Aut(FA); and

BT) : Aut(FdI)→Aut(FAT).

(3.1.7)

This gives us a new interpretation of the choice of groups G and GT: both are pushforwards of subgroups of Aut(FdI) via the Shioda maps φB and φBT,

respec-tively.

### Reinterpretation of the Dual Group

We now reformulate the relationship between the groups G and GT via a bilinear pairing. Consider the map

h,iB :Zn+1×Zn+1 −→Z

where hs,hiB :=sTBh. Choose a subgroupG⊂Aut(FA), so thatJFA ⊆G. Then

set H := ((φB)∗)−1(G). Note that the map (hj)j 7→ (e2πihj/d)j induces a natural, surjective group homomorphism

Zn+1 →pr Aut(FdI). (3.1.8)

Take ˜H to be the inverse image ˜H :=pr−1(H) of H under this map. We can then define the subgroup ˜H⊥B ⊆

Zn+1 to be

˜

H⊥B := n

s∈Zn+1

hs,hiB ∈dZ for all h ∈

˜

Ho. (3.1.9)

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We remark that it is clear that the groupJFdI is contained byH as

(φB)∗(e2πi/d, . . . , e2πi/d) =

Y

j

ρj (3.1.10)

is a generator of JFA.

We have assumed that the groupGis a subgroup ofSL(FA). This requires that, for all group elements h= (hk)k ∈H˜, the product Qn

j=0e

2πi d

P

kbjkhk equals 1. This

implies that the sumPn

j,k=0bjkhkis an integer divisible by d; therefore, (1, . . . ,1)∈ ˜

H⊥B. So, its image pr(1, . . . ,1) must be in H⊥B. The element pr(1, . . . ,1) =

(e2πi/d, . . . , e2πi/d) is a generator of the group JFdI, henceH

⊥B contains J

FdI.

Moreover, if one unravels all the definitions, one can see that (φBT)(H⊥B) = GT. In order for a monomial Qn

i=0x si

i to be G-invariant, we will need, for any

Qn

i=1ρ hi

i = (e

2πi d

Pn

i=0bijhj)i G, that Qn

i=0x si

i =

Qn

i=0(e

2πi d

Pn

i=0bijhjxi)si. This is

equivalent to P

i,jsibijhj being a multiple of d.

### Birational Geometry of BHK Mirrors

We now give a Theorem of Bini, written in our notation (Theorem 3.1 of ):

Theorem 3.1.1. Let all the notation be as above. Then the hypersurfaces XA and

XAT are birational to the quotients of the Fermat varietyXdI/(((φB))−1(JFA)/JFdI)

and XdI/(((φBT))−1(JF

AT)/JFdI), respectively.

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via composing φB with the map

qA:WPn(q0, . . . , qn)99KPn+1;

(x0 :. . .:xn)7−→

Y

j

xj :

Y

j

xa1j

j :. . .:

Y

j

xanj

j

!

.

(3.1.11)

Note that the composition qA◦φB :XdI 99KPn+1 gives the map

(y0 :. . .:yn)7−→

Y

j

yq

0

j

j :y d

1 :. . .:ydn

!

. (3.1.12)

Letting m= gcd(d, q10, . . . , qn0), we describe the closure of the image as

MA :=Z n

X

i=1

ui, u d/m

0 =

n

Y

i=1

uq0i/m

i

!

Pn+1. (3.1.13)

Bini then proves that the mapqA◦φB is birational to a quotient map, which in our notation implies the birational equivalence

MA'XdI/(φ−B1(SL(FA))/JFdI). (3.1.14)

Bini then refers the reader to the proof of Theorem 2.6 in  to see why the other two maps are birational to quotient maps as well. Note that Bini requires

d to be the smallest positive integer so that dA−1 is an integral matrix, but the requirement that dis the smallest such integer is unnecessary. One can just use the first part of Theorem 2.6 of  to eliminate this hypothesis.

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XA/G¯ is birational to XdI/(((φB)∗)−1(JFA) +H/JFdI) while XAT/G¯

T is birational

to XdI/(((φBT))−1(JF

AT) +H

⊥B/J

FdI). As JFA ⊆G and JFAT ⊆G

T,

((φB)∗)−1(JFA)⊆((φB)∗)

−1(G) =H; and

((φBT))−1(JF

AT)⊆((φBT)∗)

−1

(GT)⊆H⊥B

(3.1.15)

which gives us the following corollary:

Corollary 3.1.2. The Calabi-Yau orbifold ZA,G is birational to XdI/(H/JFdI) and

its BHK mirror ZAT,GT is birational to XdI/(H⊥B/JF dI).

### Multiple Mirrors

As stated in the introduction, one can take two polynomials

FA = n X i=0 n Y j=0

xaij

j ; FA0 =

n X i=0 n Y j=0 xa 0 ij

j . (3.2.1)

that cut out two hypersurfaces in weighted-projective n-spaces,

XA⊆WPn(q0, . . . , qn) and

XAT ⊆WPn(q00, . . . , q00),

(3.2.2)

respectively. Take two groups G and G0 so that JFA ⊆ G ⊆ SL(FA) and JFAT ⊆ GT SL(F

AT). We then obtain two Calabi-Yau orbifolds ZA,G := XA/G¯ and ZA0,G0 :=XA0/G¯0.

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n-space (see Section 5 for an explicit example or Tables 5.1-3 of ). Take the polynomials

FAT =

n X i=0 n Y j=0

xaji

j ; F(A0)T =

n X i=0 n Y j=0 xa 0 ji

j . (3.2.3)

They are quasihomogeneous polynomials but not necessarily with the same weights. So they cut out hypersurfaces XAT and X(A0)T. Take the dual groups GT and

(G0)T toGand G0, respectively. We quotient each hypersurface by their respective groups, ¯GT :=GT/J

FAT and ( ¯G0)T := (G0)T/JF(A0)T. We then have the BHK mirror

dualities:

ZA,G

BHK mirrors

←→ ZAT,GT

ZA0,G0 BHK mirrors←→ Z(A0)T,(G0)T

(3.2.4)

In this section, we will investigate and compare the birational geometry of the BHK mirrors of the Calabi-Yau orbifoldsZA,GandZA0,G0 by using the Shioda maps.

Take positive integers d and d0 so that B :=dA−1 and B0 :=d0(A0)−1 are matrices with integer entries. Then we can form a “tree” diagram of Shioda maps:

Xdd0I

φd0I

u

u

k k k k

k k k k k φdI ) ) S S S S S S S S XdI φB } } { {

{{ φBT

"

"

E E E

E Xd0I

φB0

|

|

y y

y y φ(B0)T

# # H H H H H

XA XAT XA0 X(A0)T

(3.2.5)

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and positive integer c. This means we can simplify our tree to just the diagram:

Xdd0I

φd0B

t

t

i i i i i i

i i i i i i

φd0BT

{

{

w ww w w

φdB0 FF""

F F

F φ

d(B0)T

* * U U U U U U U U U U U

XA XAT XA0 X(A0)T

(3.2.6)

The Calabi-Yau orbifolds are just finite quotients of the hypersurfacesXA,XAT, XA0 and X(A0)T, so we can view them in the context of the diagram:

Xdd0I

φd0B

s

s

h h h h h h

h h h h h h h

φd0BT

z

z

u uu u u

φdB0 HH\$\$

H H

H φ

d(B0)T

+ + W W W W W W W W W W W W W XA

XAT

XA0

X(A0)T

ZA,G ZAT,GT ZA0,G0 Z(A0)T,(G0)T

(3.2.7)

Letting H and H0 be the groups H := (φd0B)−1(G) and H0 := (φdB0)−1(G0), we

know that:

Proposition 3.2.1. The following birational equivalences hold:

ZA,G 'Xdd0I/(H/JF

dd0I); ZAT,GT 'Xdd0I/(H⊥d0B/JF

dd0I); ZA0,G0 'Xdd0I/(H0/JF

dd0I); and Z(A0)T,(G0)T 'Xdd0I/((H0)⊥dB0/J

Fdd0I).

(3.2.8)

Proof. Follows directly from Corollary 3.1.2.

Recall that we are asking for the conditions in whichZAT,GT and Z(A0),(G0)T are

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and (H0)⊥dB0 are equal. We now prove that such an equality holds, if we assume

that the groups G and G0 are equal.

Lemma 3.2.2. If the groups G and G0 are equal, then H⊥d0B and (H0)⊥dB0 are

equal.

Proof. Set ˜H := pr−1(H) and ˜H0 := pr−1(H0) (Recall these groups from Section 3.2). Note that we have an equality of groups (φd0B)◦pr( ˜H) = G=G0 = (φdB0)

pr( ˜H0). This implies that, for any element h∈H˜, there exists an element h0 ∈H˜0

so that d0Bh=dB0h0.

Suppose that s∈ ( ˜H0)⊥dB0. We claim that s is in ˜H⊥d0B, i.e., for every h ∈ H˜,

that hs,hid0B ∈ dZ. Indeed, this is true. Given any h ∈ H˜, there exists some h0

as above where d0Bh = dB0h0, hence hs,hid0B = hs,h0idB0 ∈ dZ, as s ∈ ( ˜H0)⊥dB0.

This proves that ( ˜H)⊥d0B ⊆ ( ˜H0)⊥dB0. By symmetry, we now have the equality of

the groups, ˜H⊥d0B = ( ˜H0)⊥dB0.

This implies that the images of the groups ˜H⊥d0B and ( ˜H0)⊥dB0 under the

homo-morphism pr are equal, hence H⊥d0B and (H0)⊥dB0 are equal.

We then have the proof of following theorem:

Theorem 3.2.3. LetZA,GandZA0,G0 be Calabi-Yau orbifolds as above. If the groups

G andG0 are equal, then the BHK mirrors ZAT,GT andZ(A0)T,(G0)T of these orbifolds

are birational.

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This answers Question 2.3.7 affirmatively in the birational case.

### Chen-Ruan Hodge Numbers

In this section, we give an example of two Calabi-Yau orbifoldsZA,G andZA0,G0 that

are in the same family, but have two different BHK mirrors ZAT,GT and Z(A0)T,(G0)T

that are not in the same family. As mentioned before, this is a feature of BHK mirror duality that differentiates it from the mirror construction of Batyrev and Borisov. We will show that the BHK mirrors are birational to each other and that their Chen-Ruan Hodge numbers match.

Consider the polynomials

FA: =y18+y 8 2 +y

4 3 +y

3 4 +y

6 5; and

FA0 : =y8

1 +y 8 2 +y

4 3 +y

3

4 +y4y54.

(3.3.1)

The polynomials FA and FA0 cut out hypersurfaces XA = Z(FA) and XA0 =

Z(FA0), two well-defined hypersurfaces in the (Gorenstein) weighted projective

4-space WP4(3,3,6,8,4). Note that they are in the same family.

We now address the groups involved in the BHK mirror construction. Set ζ

to be a primitive 24th root of unity. The groups JFA and JFA0 are equal and are

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same group, namely we define it to be

G=G0 :=h(ζ3, ζ3, ζ6, ζ8, ζ4),(ζ18,1, ζ6,1,1),(1,1, ζ12,1, ζ12)i. (3.3.2)

Note that each of the generators of the group G are also in SL(FA) and SL(FA0),

hence the group G satisfies the conditions required for BHK duality. We quotient both the hypersurfaces by XA and XA0 by the group G/JF

A to obtain the

Calabi-Yau orbifolds ZA,G and ZA0,G which are in the same family of hypersurfaces in

WP4(3,3,6,8,4)/(G/J FA).

### BHK Mirrors

Next, we describe the BHK mirrors toZA,G andZA0,G0. The polynomials associated

to the matrices A and AT are

FAT =FA: =y18+y28+y34+y34 +y65; and

FA0T : =z81 +z28+z34+z34z5+z45.

(3.3.3)

While the hypersurface XAT = Z(FAT) is in WP4(3,3,6,8,4), the hypersurface X(A0)T =Z(FA0T) is in a different (Gorenstein) weighted projective 4-space, namely WP4(1,1,2,2,2). We can compute the following groups:

JFAT =h(ζ3, ζ3, ζ6, ζ8, ζ4)i;

JF(A0)T =h(ζ

3, ζ3, ζ6, ζ6, ζ6)i;

GT =h(ζ3, ζ3, ζ6, ζ8, ζ4)i; and

(G0)T =h(ζ3, ζ3, ζ6, ζ6, ζ6),(1,1,1, ζ12, ζ12)i.

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Note that the groupsGT and J

FAT are equal, so the BHK mirrorZAT,GT is the

hypersurfaceXAT. On the other hand the quotient group (G0)T/JF

(A0)T is isomorphic

toZ2, hence the BHK mirrorZ(A0)T,(G0)T is the Calabi-Yau orbifoldX(A0)T/Z2. Note

that the Calabi-Yau orbifold ZAT,GT is a hypersurface in WP4(3,3,6,8,4), while Z(A0)T,(G0)T is in WP(1,1,2,2,2)/Z2. The two BHK mirrors are not hypersurfaces

of the same quotient of weighted-projective spaces, hence not sitting inside the same family of Calabi-Yau orbifolds.

### Shioda Maps

Even though the two BHK mirrors ZAT,GT and Z(A0)T,(G0)T do not sit in the same

family of hypersurfaces of the same quotient of weighted-projective space, we can show that they are birational. Take the matrices B := 24A−1 and B0 := 24(A0)−1. Let X24I be the hypersurface Z(x241 +x242 +x243 +x244 +x245 ) ⊂ P4. We then have the Shioda maps

X24I φB t t iiiiiiiiii iiiiiiiiii iii φBT | | xxxxxx xxx

φB0 EE""

E E

E φ

(B0)T

* * U U U U U U U U U U U

XA XAT XA0 X(A0)T

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The maps then can be described explicitly:

φB(x1 :. . .:x5) = (x31 :x 3 2 :x

6 3 :x

8 4 :x

4

5)∈XA

φBT(x1 :. . .:x5) = (x31 :x23 :x63 :x84 :x45)∈XAT

φB0(x1 :. . .:x5) = (x3

1 :x 3 2 :x

6 3 :x

8 4 :x

−2 4 x

6

5)∈XA0

φ(B0)T(x1 :. . .:x5) = (x31 :x32 :x63 :x84x−52 :x65)∈X(A0)T

(3.3.6)

The four Shioda maps are rational maps that are birational to quotient maps. Take the following four subgroups to Aut(F24I):

H :=(ζ, ζ, ζ, ζ, ζ),(ζ8,1,1,1,1),(ζ2,1, ζ−1,1,1),(1,1, ζ2,1, ζ3),

(1,1,1, ζ, ζ4)

;

H0 :=(ζ, ζ, ζ, ζ, ζ),(ζ8,1,1,1,1),(ζ2,1, ζ−1,1,1),(1,1, ζ2,1, ζ2),

(1,1,1, ζ3, ζ);

H⊥B = (H0)⊥B0 :=(ζ, ζ, ζ, ζ, ζ),(ζ8,1,1,1,1),(1,1, ζ4,1,1),(ζ2, ζ2, ζ2,1,1),

(1,1,1, ζ, ζ4);

JF24I =

(ζ, ζ, ζ, ζ, ζ) .

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By Proposition 3.2.1, we have the following birational equivalences

ZA,G 'X24I

(ζ8,1,1,1,1),(ζ2,1, ζ−1,1,1),(1,1, ζ2,1, ζ3),(1,1,1, ζ, ζ4);

ZA0,G0 'X24I

(ζ8,1,1,1,1),(ζ2,1, ζ−1,1,1),(1,1, ζ2,1, ζ2),(1,1,1, ζ3, ζ);

ZAT,GT 'X24I

(ζ8,1,1,1,1),(1,1, ζ4,1,1),(ζ2, ζ2, ζ2,1,1),(1,1,1, ζ, ζ4)

;

Z(A0)T,(G0)T 'X24I

(ζ8,1,1,1,1),(1,1, ζ4,1,1),(ζ2, ζ2, ζ2,1,1),(1,1,1, ζ, ζ4).

(3.3.8) So we can see that the BHK mirrors ZAT,GT and Z(A0)T,(G0)T are birational.

### Chen-Ruan Hodge Numbers

As the Calabi-Yau orbifolds ZA,G and ZA0,G are quasismooth varieties in the same

toric variety, namely WP4(3,3,6,8,4)/h(i,1, i,1,1),(1,1,1,1,1)i, they have the same Chen-Ruan Hodge numbers. By the theorem of Chiodo and Ruan, this means that their BHK mirrors ZAT,GT and Z(A0)T,(G0)T must have the same

Chen-Ruan Hodge numbers. We now check this explicitly.

Consider the hypersurfaceXAT ⊆WP4(3,3,6,8,4). The dual groupGT is equal

to the group JFAT. The only elements of the group GTC∗ that will have nontrivial fixed loci are in JFAT as the weighted projective space is Gorenstein. The group

JFAT has exactly six elements which have fixed loci that have nonempty intersections with the hypersurface (see Table 3.1).

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Element of JFAT Fixed Locus (1,1,1,1,1) XAT

(ζ18, ζ18, ζ12,1,1) Z(y1, y2, y3)∩XAT

(1,1,1, ζ16, ζ8) Z(y4, y5)∩XAT

(ζ12, ζ12,1,1,1) Z(y1, y2)∩XAT

(1,1,1, ζ8, ζ16) Z(y

4, y5)∩XAT

(ζ6, ζ6, ζ12,1,1) Z(y

1, y2, y3)∩XAT

Table 3.1: Elements of JFAT with fixed loci

diamond of:

1

0 0

0 1 0

1 36 36 1

0 1 0

0 0

1

The remaining fixed loci are simpler: Z(y1, y2, y3)∩XAT consists of three points, Z(y4, y5)∩XAT is a curve of genus nine, and Z(y1, y2)∩XAT is a curve of genus

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the Calabi-Yau orbifold ZA,G:

1

0 0

0 7 0

1 55 55 1

0 7 0

0 0

1

Next, we check that these are the same Chen-Ruan Hodge numbers as the Calabi-Yau orbifold Z(A0)T,(G0)T. Recall that XA0T ⊂ WP4(1,1,2,2,2), so we will

have a different C∗ action. The group (G0)T is the group JF(A0)T · h(1,1,1,−1,−1)i.

As the weighted-projective space is Gorenstein, we can only look at (G0)T to find the nontrivial fixed loci of elements. The group (G0)T only has five elements that will have nonempty intersections between the hypersurface and the fixed loci of the elements (see Table 3.2).

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)-Element of (G0)T Fixed Locus (1,1,1,1,1) X(A0)T

(ζ12, ζ12,1,1,1) Z(z1, z2)∩X(A0)T

(1,1,1, ζ12, ζ12) Z(z4, z5)∩X(A0)T

(ζ6, ζ6, ζ12,1,1) Z(z1, z2, z3)∩X(A0)T

(ζ18, ζ18, ζ12,1,1) Z(z

1, z2, z3)∩X(A0)T

Table 3.2: Elements of (G0)T with fixed loci

invariant part of the cohomology of X(A0)T gives the Hodge diamond:

1

0 0

0 1 0

1 45 45 1

0 1 0

0 0

1

Z(z1, z2)∩X(A0)T is a curve with a Z2 invariant h0,1 = 1, Z(z4, z5)∩X(A0)T is a Z2

-invariant curve of genus nine, and Z(z1, z2, z3)∩X(A0)T is a set of four Z2-invariant

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of Z(A0)T,(G0)T:

1

0 0

0 7 0

1 55 55 1

0 7 0

0 0

1

Note that this Chen-Ruan Hodge diamond matches that of the Calabi-Yau orb-ifoldZA,G. To summarize, what we have given here is two Calabi-Yau orbifoldsZA,G and ZA0,G0 that live in a family of hypersurfaces in a finite quotient of a

weighted-projective space. Their BHK mirrors ZAT,GT and Z(A0)T,(G0)T do not sit in a single

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## of BHK-Type

### Introduction

A classical problem is to compute the Picard rank of a given algebraic surface. Much work has been done in recent years in order to understand a generic hypersurface in a toric Fano 3-fold , but one may ask about the highly symmetric hypersurfaces in a weighted projective space. In this note, we will give an explicit description in order to compute the Picard rank of certain symmetric K3 surfaces that are hypersurfaces in weighted projective spaces. This is a generalization of the case of Delsarte surfaces answered by Shioda in .

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generalization to the Delsarte surfaces in the K3 case are K3 surfaces of BHK-type, that is, K3 surfaces that have a Berglund-H¨ubsch-Krawitz mirror. Let k be an algebraically closed field. Take FA is a polynomial that is a sum of four monomials with four variables over k

FA:= n

X

i=0 n

Y

j=0

xaij

j (4.1.1)

where A is an invertible matrix with entries (aij)ni,j=0. Assume that FA cuts out a quasismooth K3 hypersurface in a weighted projective space XA := Z(FA) ⊆

WPn(q

0, . . . , qn) for some positive integersqi. This implies that the weighted degree

d of the polynomial FA is the sum

P

iqi. The group (k∗)n+1 acts on the space W

Pn(q0, . . . , qn) by coordinate-wise multi-plication with a subgroupk∗ that acts trivially. Define the groupSL(FA) to be the elements of (k∗)n+1 that preserve the polynomial FA and the holomorphic 3-form. Choose a subgroup G of SL(FA) such that it contains k∗ ∩SL(FA). Setting ¯G to be G/(SL(FA)∩k∗), the orbifold quotient ZA,G:=XA/G¯ is a K3 orbifold.

Berglund-H¨ubsch-Krawitz mirror symmetry proposes that the mirror to the orb-ifold ZA,G is related to the polynomial associated to the transposed matrix AT,

FAT := Pni=0Qnj=0z

aji

j . The polynomial FAT cuts out a Calabi-Yau

hypersur-face XAT := Z(FAT) ⊆ WPn(r0, . . . , rn) for some positive integers ri. The dual

group introduced by Krawitz in  is a group GT which satisfies the analogous conditions for XAT that the group G does for XA. Define the quotient group

¯

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ZA,G to be the quotient XAT/G¯T.

There has been a flurry of activity on BHK mirrors. Chiodo and Ruan in  prove that the mirror symmetry theorem for these (n−1)-dimensional Calabi-Yau orbifolds on the level of Chen-Ruan Hodge cohomology:

HCRp,q(ZA,G,C) =HCR(n−1)−p,q(ZAT,GT,C).

This is evidence that the orbifoldsZA,GandZAT,GT form a mirror pair in dimensions

3 and greater; however it does not tell us anything in the case of surfaces. There has been recent work of Artebani, Boissi`ere and Sarti that tries to unify the BHK mirror story with Dolgachev-Voisin mirror symmetry in the case where the hypersurface

XA is a double cover of P2. Their work has been extended by Comparin, Lyons, Priddis, and Suggs to prime covers of P2. In this corpus of work, the authors focus on proving that the Picard groups of the K3 surfaces ZA,G and ZAT,GT have

polarizations by so-called mirror lattices. In particular, these lattices embed into the subgroup of the Picard groups of the BHK mirrors that are invariant under the non-symplectic automorphism induced on the K3 surface due to the fact of it being a prime cover of P2. The fact that it is a prime cover of

P2 requires that the polynomial look like

FA:=xa000 + n

X

i=1 n

Y

j=1

xaij

j . (4.1.2)

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are relating each surface of BHK-type birationally to a quotient of a higher degree Fermat hypersurface in projective space by a finite group H. We then describe the

H-invariant part of the transcendental lattice of the Fermat hypersurface, which gives us the rank of the transcendental lattice of the surface of BHK type and the Lefschetz number.

Take BHK mirrors surfaces ZA,G and ZAT,GT as above. Take d to be a positive

integer so that the matrix dA−1 has only integer entries. Let I

d be the subset of symmetries on a degree d Fermat hypersurface Xd that correspond to elements in the transcendental lattice of Xd tensored with Q, Tn(Xd) (See Section 3.1 for an explicit description of Id that is very computable). We remark that the subset Id depends on the characteristic of the fieldk. We then describe the rank of the Picard group. In particular, we prove the following theorem:

Theorem 4.1.1. The Lefschetz numbers of the BHK mirrorsZA,G and ZAT,GT are: λ(ZA,G) = #(Id∩GT) and

λ(ZAT,GT) = #(Id∩G).

(4.1.3)

The surprise is that the dual group GT associated to the BHK Mirror ZAT,GT

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Corollary 4.1.2. The Picard ranks of the BHK mirrors ZA,G and ZAT,GT are: ρ(ZA,G) = 22−#(Id∩GT) and

ρ(ZAT,GT) = 22−#(Id∩G).

(4.1.4)

An added quick corollary is a lower bound on the Picard number of a BHK mirror is by the order of dual group GT. Also, a great benefit to this is that the Picard number of each BHK mirror surface is now computable, once one chooses over which field one works.

### Surfaces of BHK-Type

In this section, we will introduce the K3 surfaces which have a BHK Mirror, so called surfaces of BHK type. We will start by introducing weighted Delsarte surfaces that are K3 surfaces and then move to quotienting by certain symplectic quotients.

### Weighted Delsarte Surfaces

Let k be an algebraically closed field. Take FA to be a sum of four monomials in four variables

FA:= 3

X

i=0 3

Y

j=0

xaij

j .

We require that the polynomial FA satisfy three conditions:

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