There are 100 prisoners, numbered 1-100. The prisoners' numbers are written onto 100 cards, put randomly into 100 envelopes numbered 1-100. They can't communicate, but are invited, one by one, to open 50 envelopes. If any prisoner fails to find his number, they all get killed.
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The sequence (or cycle) each prisoner lands may have length from 1 to 100, and there may be anything from 1 to 100 such sequences. But every prisoner is using the same set of cycles. This strategy means that the fates of the prisoners are no longer independent.
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The probability that there's a cycle (there could only ever be at most one) longer than fifty turns out to be about 70%, and that corresponds to a prisoner needing to open more than fifty envelopes to get back to their number.
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