Euler's totient function in F#

totient.fsx

let totient n =
    let rec totient isFirst (r, n) d =
        match n % d with
        | 0 -> totient false (r * (d - (if isFirst then 1 else 0)), n / d) d
        | _ -> r, n
    let nearlyPrimes = seq { yield 2; for i in 3 .. 2 .. n do yield i }
    ((1, n), nearlyPrimes) ||> Seq.fold (totient true) |> fst

let totientTest() =
    let expected = [1; 1; 2; 2; 4; 2; 6; 4; 6; 4]
    let actual = [for i in 1 .. 10 -> totient i]
    if actual <> expected then failwith "expected : %A, but : %A" expected actual

References : http://sucrose.hatenablog.com/entry/2014/04/22/005557