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Counting k between 100 and 400 with k to the k square

Question

For how many integers kk with 100≤k≤400100\le k\le 400 is the number kkk^k a perfect square?

Short answer

156156. If kk is even, kk=(kk/2)2k^k=(k^{k/2})^2, giving 151151 values. If kk is odd, kkk^k is a square exactly when kk itself is a square; the odd squares in range are 121,169,225,289,361121,169,225,289,361, adding 55.

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