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.

- Code: Select all
`Integer method: properDivs { #[ self swap mod not ] self 2 / seq filter }`

: amicables(n) // ( n -- aList ) : Returns list for pairs of amicables under n.

{

| i j |

ListBuffer new

n loop: i [

i properDivs sum dup ->j i <= ifTrue: [ continue ]

j properDivs sum i <> ifTrue: [ continue ]

[ i, j ] over add

]

}

: pe021 { 0 10000 amicables apply(#[ sum + ]) }