You have a pack of 52 playing cards. • Shuffle the cards. • Pick a card at random. • Note what it is. • Return it to the deck. • If you’ve now seen every card, you’re done. • Otherwise, repeat. What is the expected number of cards picked, rounded to the nearest integer?
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Replying to @gregeganSF
It's annoying how hard this is for me. After some despair I decide to think about what happens when I've seen 51 of the cards and am waiting for the last. I keep picking cards; each time with probability 51/52 I fail to get the one I want. The expected time to success? (1/n)
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Replying to @johncarlosbaez @gregeganSF
The probability that I succeed on the nth try is the probability I fail (n-1) times and then succeed: (1/52)(51/52)^{n-1} = 51/52^n. Sanity check: these probabilities sum to 1. So the expected time to success is the sum of n (51/52)^n from n=1 to infinity. (2/n)
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Replying to @johncarlosbaez @gregeganSF
The sum of nx^n from 1 to infinity is x times the sum of nx^(n-1) from 1 to infinity. So it's x times the derivative of 1/(1-x), i.e. x/(1-x)^2. Taking x = 51/52 I get (51/52) / (1/52)^2 = 51 times 52 (3/n)
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Replying to @johncarlosbaez @gregeganSF
So if I haven't screwed up the expected number of tries to get the *last* card is 51 times 52 = 2652. I should be able to compute the expected number of tries to get the second to last card, etc., and add these up to get the answer. Bed-time. (4/n)
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