Function: polsubcyclo
Section: polynomials
C-Name: polsubcyclo
Prototype: LLDn
Help: polsubcyclo(n,d,{v='x}): finds an equation (in variable v) for the d-th
 degree subfields of Q(zeta_n). Output is a polynomial, or a vector of
 polynomials if there are several such fields or none.
Doc: gives polynomials (in variable $v$) defining the sub-Abelian extensions
 of degree $d$ of the cyclotomic field $\Q(\zeta_n)$, where $d\mid \phi(n)$.

 If there is exactly one such extension the output is a polynomial, else it is
 a vector of polynomials, possibly empty. To get a vector in all cases,
 use \kbd{concat([], polsubcyclo(n,d))}.

 The function \tet{galoissubcyclo} allows to specify exactly which
 sub-Abelian extension should be computed.
