Problem 565
Divisibility of sum of divisors
Let
E.g. the divisors of 4 are 1, 2 and 4, so
The numbers n not exceeding 20 such that 7 divides
Let S(n,d) be the sum of the numbers i not exceeding n such that d divides
So S(20,7)=49.
You are given:
Find
约数和的整除性
记
例如4的约数为1,2和4,因此
在不超过20的数n中,
记S(n,d)是所有不超过n且
因此S(20,7)=49。
已知
求
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