# 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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## closed as off-topic by Kevin P. Costello, Andres Caicedo, Andrey Rekalo, Ricardo Andrade, SuvritSep 25 '13 at 21:43

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question does not appear to be about research level mathematics within the scope defined in the help center." – Kevin P. Costello, Andres Caicedo, Andrey Rekalo, Ricardo Andrade, Suvrit
If this question can be reworded to fit the rules in the help center, please edit the question.

I've spent 4 hours on this problem using Mathematica and only came up with a slow numeric solution. –  cool-RR Sep 25 '13 at 18:37
Check out: math.stackexchange.com/questions/72223/… –  Benjamin Dickman Sep 25 '13 at 19:34
Found an answer here: math.stackexchange.com/questions/41519/… Thanks! –  cool-RR Sep 25 '13 at 20:07