primeFactors.fs

//with mutation
let Primes number = 
    let mutable factors = []
    let mutable n, factor = number, 2
    while n > 1 do
        if n % factor = 0 then 
            factors <- List.append factors [factor]
            n <- n / factor
        else factor <- factor + 1
    factors

//without mutation
let Primes number = 
    let rec factorsOf number factor accumulator = 
        if number > 1 && number % factor = 0 then 
            factorsOf (number / factor) (factor) (List.append accumulator [factor])
        else if number > 1 then 
            factorsOf (number) (factor + 1) (accumulator)
        else accumulator
    factorsOf number 2 []