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数论（欧拉降幂） ——杭电多校赛第九场E 给定 a ,b ,c , d , x, y 求 0⩽a,b,c,d⩽3×106,0<x,y⩽109,a⩽b,c⩽d. 首先对问题转化为枚举质因子，计算为每个公共质因子贡献的问题。规模为O（log x + log y）乘积就转化成了和的形式只需求出此式（指数）然后快速幂即可。 这里还要用到一个原理就是指数可能会爆long long 所以就是用到一个欧拉降幂，原理我也是看不懂，但是公式记住先。在快速幂取模时指数对mod-1取余，其他还是不变对mod取余。或者这里

环+逆元 ——牛客多校赛第二场J题 Given a permutation with size n and an integerk, you should find a permutation substitutionP that{1,2,⋯,n} will become {A}A after performing substitution P for exactly k times. Print the permutation after performing P for once on{1,2,⋯,n}

数论 ——Dead Fraction（POJ - 1930 ） Mike is frantically scrambling to finish his thesis at the last minute. He needs to assemble all his research notes into vaguely coherent form in the next 3 days. Unfortunately, he notices that he had been extremely sloppy i