Alexander Grothendieck is one of the most famous and influential mathematicians of the 20th century, having largely single-handedly invented modern algebraic geometry.
He was fond of working in generality rather than in specific examples. Once during a talk, he said "let p be a prime" and so on until a member of the audience asked him to pick a particular prime for the purposes of illustration.
"Okay, let p be 57."
Someone in the audience had to point out that 57 = 3 * 19.
Grothendieck "died" in the sense meant by Paul Erdos (stopped doing new mathematical research) at a surprisingly young age, while Israel Gelfand kept going into his nineties, not "dying" (ceasing to be a mathematician) until he "left" (died in the usual sense of the term).
Gelfand poses delightful problems that give students a workout in arithmetic (and CAN'T be done with a calculator) and that build conceptual understanding and interest in higher mathematics.
He was fond of working in generality rather than in specific examples. Once during a talk, he said "let p be a prime" and so on until a member of the audience asked him to pick a particular prime for the purposes of illustration.
"Okay, let p be 57."
Someone in the audience had to point out that 57 = 3 * 19.