For a first round phone interview for an engineering internship at Google, I was asked to solve a reasonably challenging question (I don't want to give it away,

For a first round phone interview for an engineering internship at Google, I was asked to solve a reasonably challenging question (I don't want to give it away, but it was algorithmic in nature and involved no code). When I got it right, the interviewer asked me to write an inductive proof of my solution and email it to him. I like doing stuff like that so I wasn't upset in the least, but I can understand why many people would find that scary and I certainly think Google's interview process is more extreme than others'. I've interviewed successfully with Goldman Sachs, Amazon, and Microsoft (among others) so that's to whom I'm comparing them.