data/problems/21.yml
---
:id: 21
:name: Amicable numbers
:url: https://projecteuler.net/problem=21
:content: "Let d(_n_) be defined as the sum of proper divisors of _n_ (numbers less
than _n_ which divide evenly into _n_). \nIf 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.\n\nFor 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.\n\nEvaluate the sum of all the amicable numbers
under 10000.\n\n"