euler21.hs

{-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.-}


import Shared(listDivisors)

euler21 = sum [a | a <- [1..9999], b <- [1..9999], a /= b, sum (init $ listDivisors a) == b, sum (init $ listDivisors b) == a]

main = print euler21 -- correct 31626 in 7.718s