This question is lifted from Mathoverflow.. I feel it belongs here too.
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.
Answer
Leonhard Euler

He made important discoveries in pretty much every mathematical field there was at his time.

He discovered graph theory.

He is responsible for much of the current mathematical notation we use today, including Σ, i, e, f(x), π, and sin/cos.

His combined works fill 80 (!) volumes.

And last but not least,
Attribution
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