ProjectEuler in F#

Problem 1 - Multiples of 3 and 5.fs

[|1..1000 - 1|] |> Array.filter (fun e -> e % 3 = 0 || e % 5 = 0) |> Array.sum

Problem 2 - Even Fibonacci numbers.fs

#nowarn "40"
let rec fibonacci = seq { yield 1; yield! Seq.scan (+) 2 fibonacci }

fibonacci |> Seq.takeWhile (fun e -> e <= 4000000) |> Seq.filter (fun e -> e % 2 = 0) |> Seq.sum

Problem 21 - Amicable numbers.fs

let numbers = [|1..10000|]
let properDivisorsSum = numbers |> Array.map(fun e1 -> e1, numbers.[0..e1/2] 
                                                           |> Array.filter(fun e2 -> e1 % e2 = 0) 
                                                           |> Array.sum)

properDivisorsSum 
|> Array.filter(fun (a,b) -> properDivisorsSum 
                             |> Array.exists(fun (c,d) -> a <> b && a = d && b = c))
|> Array.sumBy(fun (a, _) -> a)

Problem 25 - 1000-digit Fibonacci number.fs

Seq.unfold(fun (i, a, b) -> Some((i, a), (i + 1, b, a + b))) (1, 1I,1I)
|> Seq.find(fun (_, e) -> (e |> string |> String.length) = 1000)
|> fst

Problem 48 - Self powers.fs

[for i in 1..1000 -> pown (bigint i) i] |> List.sum |> (fun e -> e % pown 10I 10)

Problem 6 - Sum square difference.fs

let square x = x * x; 
let range = [|1..100|];

(range |> Array.sum |> square) - (range |> Array.map square |> Array.sum)