## Project Euler : Problem 021

Project Euler problems in Oforth . Feel free to post your own code.

### Project Euler : Problem 021

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.

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 + ]) }`
Franck

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Joined: 29 Oct 2014 19:01

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