Euler 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.