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Expected peaks in a random ordering of 1 to 14

Question

The numbers 1,2,…,141,2,\dots,14 are arranged in a uniformly random order in a row. Call a position a peak if its number exceeds every number in the neighbouring position(s); the two end positions have just one neighbour each. For instance, the ordering 513246513246 of 11–66 has peaks at positions 11, 33 and 66. What is the expected number of peaks?

Short answer

55. Use indicators and linearity of expectation. An end position is a peak with probability 1/21/2 (it must beat its single neighbour) and an interior position with probability 1/31/3 (it must be the largest of three). So the mean is 2⋅12+12⋅13=52\cdot\tfrac12+12\cdot\tfrac13=5.

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

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