title: Evaluate the sum of all amicable pairs under 10000.
url: http://projecteuler.net/problem=21

desc: |
  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.
  
solution: Bruteforce

todo: Implement caching in other solutions

solutions:
  solve.php:
    desc: (Uses a cache to find allready calculated values for d()
    language: php
  solve.rb:
    desc: Basic solution
    language: ruby
  solve.c:
    desc: ANSI C solution (Compiled with TCC)
    language: c
