Project Euler 1, 2 and 3: Functional Programming style using F#

Question

After doing about 50 Project Euler problems in my "native" language (C#) I decided to redo some of the problems I did many years ago in F#, as it's a language I always want to give a go. Judging from the code review meta it seems this is okay to post here: Project Euler Solutions

My question is not about how I'm solving the problem, but about how I'm using F# for my chosen solution. I'm a big fan of short, concise code, but readability should be taken into mind.

Multiples of 3 or 5

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000.

    let isMultipleOf y x =
      x % y = 0

    let isMultipleOf3Or5 n =
      isMultipleOf 3 n || isMultipleOf 5 n

    seq { 1..999 }
    |> Seq.filter isMultipleOf3Or5
    |> Seq.sum

Even Fibonacci numbers

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

    let fib n =
      let rec loop zero one n =
        match n with
        | 0 -> zero
        | 1 -> one
        | _ -> loop one (zero + one) (n - 1)
      loop 1 2 n

    let isEven n =
      n % 2 = 0

    let isBelow x y =
      x >= y

    Seq.initInfinite fib
    |> Seq.takeWhile (isBelow 4000000)
    |> Seq.filter isEven
    |> Seq.sum

Largest prime factor

The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ?

    let isMultipleOf x y =
      x % y = 0L

    let getFirstFactor n =
      { 2L..n }
      |> Seq.filter (isMultipleOf n)
      |> Seq.head

    let rec getFactors n =
      seq {
        let factor = getFirstFactor n
        yield factor
        if n <> factor then yield! getFactors (n / factor) 
      }

    600851475143L |> primefactors |> Seq.max |> int