# Amicable numbers

*Author: Gerhard R*

https://projecteuler.net/problem=21

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.

Source code: prob021-gerdr.pl

use v6; sub d(Int $n) { my $sum = 1; my $sqrt-n = sqrt $n; for 2..Int($sqrt-n) -> $a { my $b = $n div $a; $sum += $a + $b if $a * $b == $n; } $sqrt-n ~~ Int ?? $sum - $sqrt-n !! $sum; } my $sum = 0; for 1..100_000 -> $a { my $b = d($a); $sum += $a + $b if $a < $b and d($b) == $a; } say $sum;