Prime and Prime factors

PrimeFactors.fs

let isprime i = 
        List.filter(fun x -> i%x = 0) [1..i]  
        |> List.length = 2

let primefactor x =
    let rec innerprime number divisor l=
        let intermedidate = number /divisor
        match (not(isprime intermedidate ),number > divisor) with
        |true,true when number % divisor = 0 ->  innerprime intermedidate divisor (divisor::l)
        |_,true when number % divisor = 0 -> intermedidate::divisor::l
        |_,true -> innerprime  number (divisor+1) l
        |_,false -> l
    innerprime x 2 []


primefactor 10
primefactor 15 
primefactor 147
primefactor 17 
primefactor 90 
primefactor 25