Pure mathemathics: authors appearing in alphabetic order
The convention in pure math is to list authors in alphabetical order.
Since this is generally known inside pure math, readers will not draw any conclusions from the author order about the relative importance of contributions. In fact, my impression is that most pure mathematicians prefer to avoid discussion of relative importance of contributions altogether, with the exception of occassionally giving more credit to their coauthors in slightly informal settings.
[As a side note, Author contribution statements seem to be unfeasible for widespread use in math. While there are collaborations where X proved Theorem 1 and Y proved Theorem 2, it is in my experience far more common that X and Y talked repeatedly over a long period of time, and at some point the proofs materialized.]
Admittedly, being listed first will mean being noticed a little bit more. However, in line with the convention about author order, the &quot;X et al&quot;-style is typically avoided. It seems far more common to refer to unwieldly large groups of authors by initials (eg GKSSW proved that...).
The place where your advisor is supposed to express how overwhelming your contributions were is in your letters of recommendation.
That said, in pure math it is also convention that merely advising a student does not establish authorship. It is even a common sentiment that advisors should be very reluctant in appearing as a coauthor on an advisor/student paper.