题目：给你一个数k判断2^(k-1)*(2^k-1)是不是完全数（真因数之和和自身相等），不是判断k是不是素数。
分析：数论。欧拉证明了所有偶完全数都满足式子2^(k-1)*(2^k-1)；其中2^k-1为素数时，上式为完全数。
满足2^k-1形式的素数叫梅森素数，这里打表计算50000内的素数判断2^k-1是不是素数即可。
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