Mathematics is made of 50 percent formulas, 50 percent proofs, and 50 percent imagination.
"A mathematician is a device for turning coffee into theorems" (P. Erdos) Addendum: American coffee is good for lemmas.
An engineer thinks that his equations are an approximation to reality. A physicist thinks reality is an approximation to his equations. A mathematician doesn't care.
Old mathematicians never die; they just lose some of their functions.
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http://www.math.utah.edu/~cherk/mathjokes.html#topic1