euler21

euler21.fsx

let factors number = seq {
    for divisor in 1 .. (float >> sqrt >> int) number do
    if number % divisor = 0 then
        yield divisor
        if number <> 1 then yield number / divisor //special case condition: when number=1 then divisor=(number/divisor), so don't repeat it
}

let d n = Seq.sum(factors n) - n
           
let all = {1..9999} |> Seq.map(fun a -> (a, d(a))) |> Map.ofSeq  
           
let amicable (all: Map<int, int>) = Seq.filter(fun (a,b) -> a<>b && all.TryFind(b) = Some a) (Map.toSeq(all))
                       
let result = all |> amicable |> Seq.map(fun (a,b) -> a) |> Seq.sum

printfn "%A" result