1月
04

12月
28

12月
20

12月
19

12月
18

# Problem 21:Amicable numbers

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

12月
17

## Project Euler个人解答(Problem 11~20)

11-20这10道题总体上不好玩，因为都是可以大量用软件自带的各种函数来计算了。。。好吧，其实不是Mathematica和Python的错。。

# Problem 11:Largest product in a grid

In the 2020 grid below, four numbers along a diagonal line have been marked in red.

12月
16

# 写在前面：

12月
08

## Euler Project之Retraction【算法优化向】

$$f_{n,a,b}(f_{n,a,b}(x))=mod(f_{n,a,b}(x),n)$$

11月
22

1.1≤p<q≤M
2.p+q≥M
3.p和q互质