# Expected number of distinct elements out of random selection [closed]

What is the expected number of distinct elements when choosing N random elements of a set of size N?

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I've spent 4 hours on this problem using Mathematica and only came up with a slow numeric solution. –  cool-RR Sep 25 at 18:37
Found an answer here: math.stackexchange.com/questions/41519/… Thanks! –  cool-RR Sep 25 at 20:07