In the first portion of this short story, a teenager and the aunt who took her in when her parents died enjoy doing math together. However, when the girl begins to get advanced training from Cambridge mathematicians and discovers a "universal equation", the story takes an interesting but decidedly nonlinear turn.
The math described in the story is never deep or interesting in itself, but there is a poetry to the author's descriptions of it that I like:
(quoted from The Central Tendency)
Sometimes I forgot what time it was, what day it was, because I was lost in numbers and how they all have different faces, depending on how you look at them. Sixteen is twice eight, or two to the power of four, or a ninth of twelve squared.
Then Lallie slowly showed me something better than numbers. She showed me tools and schemes. Matrices and transformations, laws, proofs, operations. Every number has a million faces, but the million faces all line up and you can cancel them all out and just be left with abstractions, blankfaced letters alone at the heart of everything.

In the same way, although I'm not sure I could describe for you what happens at the end  to me it's sort of like a recursive definition that regresses infinitely  there is still something emotionally satisfying about it.
This story was published in the July 2003 issue of Strange Horizons and fortunately is still available online.
