Project Euler F#

1to5.fs

//problem 1
let rec problem1 n =
    match n with
    | 0 -> 0
    | i -> if (i%3 = 0)||(i%5 = 0) then i + problem1(n-1) else problem1(i-1) 

//problem 2
let rec problem2 first second =
    if first > 4000000 
        then 0
        else if first%2 = 0 
            then first + (problem2 (first+second) first) 
            else problem2 (first+second) first

//problem 3
let rec problem3 (n:int64) (p:int64) =
    if n = p 
        then n
        else if n%p = 0L
            then problem3 (n/p) p
            else problem3 n (p+2L)

//problem 4
let rec problem4il n = 
    if n < 10
        then 0
        else 1 + (problem4il (n/10))

let rec problem4i n m =
    if (problem4il n) <> m then false
    else if n < 10
        then true
        else
            let t =  (pown 10 (problem4il n))
            if (n%10) = (n/t)
            then problem4i (((n - (n/t)*t))/10) (m-2)
            else false
    
let rec problem4 first second=
    if second = 1000
        then 0
        else if first 
            = 1000
            then problem4 (second+1) (second+1)
            else if problem4i (first*second) (problem4il (first*second))
                then max (problem4 (first+1) second) (first*second)
                else problem4 (first+1) second

//problem 5
let rec problem5i n i =
    if i = 0 then true
    else (n%i = 0) && (problem5i n (i-1))

let rec problem5 n =
    if problem5i n 20 then n else problem5 (n + 2)

[<EntryPoint>]
let main argv = 
    System.Console.WriteLine(problem1 999)
    System.Console.WriteLine(problem2 2 1)
    System.Console.WriteLine(problem3 600851475143L 3L)
    System.Console.WriteLine(problem4 100 100)
    System.Console.WriteLine(problem5 20)
    0 // 整数の終了コードを返します