Right, your program is 2 times faster than mine on my machine... I wonder if there is a better structure to do this bookkeeping than IntSet (maybe Sequence slightly remanied ?), anyway it goes to show how sometimes the bookkeeping can be more expensive than the operations it's meant to prevent ! As for my sumOfProperDivisors function it's dead simple, I would even say stupid (but it's faster than anything else I tried for now). An accumArray use the function you give it to update a cell, ok ? Here it's just (+) and all cells began their life as 1 since 1 is a proper divisors of all numbers (except 1, thus the (1,-1) for correctness). The following list just associate each proper divisor of a num with it, so the final value of a cell is the sum of those proper divisors. To achieve that we make factor take the value of all proper divisors possible for numbers from 1 to 1000000, in other words [2..1000000 `div` 2] (the `div` 2 is ok since we're speaking about proper divisors here), and then we go on to associate this divisor with all the numbers he divides properly, which are [factor * 2, factor * 3,...1000000]. Is it clear now ? -- Jedaï