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Waiting for a decreasing run to break

Question

Let X1,X2,…X_1, X_2, \dots be i.i.d. draws from a continuous distribution. Define NN as the first index at which the sequence goes up, i.e. X1≥X2≥⋯≥XN−1X_1\ge X_2\ge\dots\ge X_{N-1} and XN−1<XNX_{N-1}<X_N. Compute ln⁡(E[N])\ln\left(\mathbb{E}[N]\right).

Short answer

11. N≥kN\ge k means the first k−1k-1 values are in decreasing order, which has probability 1(k−1)!\tfrac{1}{(k-1)!} since all orderings are equally likely. Summing tail probabilities, E[N]=∑k≥11(k−1)!=e\mathbb{E}[N]=\sum_{k\ge1}\tfrac1{(k-1)!}=e, so the log is 11.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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