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Ceiling of a uniform ratio being a perfect square

Question

Let XX and YY be independent Unif(0,1)\text{Unif}(0,1) random variables. The chance that the integer ⌈Y/X⌉\lceil Y/X\rceil equals a perfect square can be written as q−π2/aq-\pi^2/a with qq rational and aa an integer. Find aqaq.

Short answer

33/233/2. The value 11 occurs when Y≤XY\le X (probability 12\tfrac12); the value k2≥4k^2\ge4 occurs on a thin triangle of area 12(1k2−1−1k2)\tfrac12\left(\tfrac{1}{k^2-1}-\tfrac{1}{k^2}\right). Using the telescoping sum ∑k≥21k2−1=34\sum_{k\ge2}\tfrac{1}{k^2-1}=\tfrac34 and ∑k≥21k2=π26−1\sum_{k\ge2}\tfrac1{k^2}=\tfrac{\pi^2}{6}-1, the total is 118−π212\tfrac{11}{8}-\tfrac{\pi^2}{12}, so aq=12⋅118aq=12\cdot\tfrac{11}{8}.

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

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