Who: G. H. Hardy

When: As a high school student

Where: The book "Pure Mathematics" -- from which I learned real analysis.

Who: Serge Lang

When: As a college student

Where: At Columbia, Serge Lang was my mathematical mentor. I took Math I C/II C from him (which I'd describe as freshman mathematics for prospective Ph.D.'s -- it was pretty much an undergraduate Abstract Algebra, plus Real Analysis plus more in two semesters). His energy and love of mathematics was inspiring. I know that nobody who met him felt neutral about him. He was incredibly dedicated to his students. If he liked you he would move mountains.

Who: Lipman Bers

When: As a college student

Where: At Columbia, Lipman Bers was my other inspiration. I took Math III C/IV C from him -- sophomore mathematics for prospective Ph.D.'s. Besides being a very lucid lecturer, with fantastic geometric intuition, he was sophisticated and kind -- a prince among men! By example he showed how one could live a mathematical life (at perhaps a bit less than the frenetic pace of Serge Lang).