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Number of divisors between k and prime p

Source: Japan Mathematical Olympiad Finals 2016 Q1

April 10, 2016
number theoryprime numbers

Problem Statement

Let pp be an odd prime number. For positive integer kk satisfying 1kp11\le k\le p-1, the number of divisors of kp+1kp+1 between kk and pp exclusive is aka_k. Find the value of a1+a2++ap1a_1+a_2+\ldots + a_{p-1}.