Expected peaks in a random ordering of 1 to 14
Question
The numbers 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 of – has peaks at positions , and . What is the expected number of peaks?
Short answer
. Use indicators and linearity of expectation. An end position is a peak with probability (it must beat its single neighbour) and an interior position with probability (it must be the largest of three). So the mean is .
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Open the worked solutionLessons behind this question
- Expectation, Mean, and Variance
- Conditional Expectation and Variance Revisited
- Sample Spaces, Events, and the Probability Axioms
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