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101 uniforms all at least 0.001 apart

Question

You draw 101101 independent uniform numbers on (0,1)(0,1). The probability that every pair of them is separated by more than 11000\tfrac1{1000} can be written in lowest terms as (ab)c\left(\tfrac ab\right)^c. Find a+b+ca+b+c.

Short answer

120120. For nn points with minimum gap dd, removing a length-dd buffer after each of the first n−1n-1 sorted points maps good configurations one-to-one onto unrestricted ones on an interval of length 1−(n−1)d1-(n-1)d. So the probability is (1−(n−1)d)n=(910)101(1-(n-1)d)^n=\left(\tfrac{9}{10}\right)^{101}, and 9+10+101=1209+10+101=120.

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

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