So I guess my problem is based on the famous coupon collector's problem, which is, if you should not be familiar with it, the following:
Given N different coupons from which coupons are being drawn independantly, with equal probability and with replacement: How many coupons do you expect to need to draw before having drawn each coupon at least once? (Wikipedia has a well written article about it).
My problem is the following: I need to know what is the expected value for each coupon after we have collected at least one copy of each coupon.
More formal: We label each of the N coupons with numbers 1,...,n. Let Xi be a random variable which counts how often we have drawn coupon i. What is E[Xi]?
Answer
The expected value of the number of purchases until you have obtained the complete collection is En=n∑i=1nn−i+1=nn∑k=11k. For reasons of symmetry E(Xi)=Enn=n∑k=11k for all i.
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