TITLE: Which mathematicians have influenced you the most?
QUESTION [98 upvotes]: There are mathematicians whose creativity, insight and taste have the power of driving anyone into a world of beautiful ideas, which can inspire the desire, even the need for doing mathematics, or can make one to confront some kind of problems, dedicate his life to a branch of math, or choose an specific research topic.
I think that this kind of force must not be underestimated; on the contrary, we have the duty to take advantage of it in order to improve the mathematical education of those who may come after us, using the work of those gifted mathematicians (and even their own words) to inspire them as they inspired ourselves.
So, I'm interested on knowing who (the mathematician), when (in which moment of your career), where (which specific work) and why this person had an impact on your way of looking at math. Like this, we will have an statistic about which mathematicians are more akin to appeal to our students at any moment of their development. Please, keep one mathematician for post, so that votes are really representative.

REPLY [5 votes]: 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).