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2022/23 AHSMC Part 1 Problem 12

Problem 12 of 16

What is the number of integers mm, 1≤m≤3001 \le m \le 300 for which mmm^m is a perfect cube?

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Solution

If m=3km = 3k then mm=(mk)3m^m = (m^k)^3 hence mmm^m is a perfect cube. If m=3k+1m = 3k+1, then mm=(mk)3mm^m = (m^k)^3 m which is a perfect cube if mm is a perfect cube. If m=3k+2m = 3k+2, then mm=(mk)3m2m^m = (m^k)^3 m^2, which is a perfect cube if mm is a perfect cube. Between 11 and 300300 there are 100100 multiples of 33, and four numbers of the form 3k+13k+1 or 3k+23k+2 which are perfect cubes (11, 8=238 = 2^3, 64=4364 = 4^3, and 125=53125 = 5^3). Therefore there are 104104 integers mm having the requested properties. The answer is (D).