Factorial primes: Difference between revisions

</pre>

=={{header|OCaml}}==

<syntaxhighlight lang="ocaml">let is_prime (_, n, _) =

let rec test x =

let d = n / x in x > d || x * d <> n && n mod (x + 2) <> 0 && test (x + 6)

in

if n < 5

then n lor 1 = 3

else n land 1 <> 0 && n mod 3 <> 0 && test 5

let factorials_plus_minus_one =

let rec next x y () =

Seq.Cons ((x, pred y, 0), Seq.cons (x, succ y, 1) (next (succ x) (succ x * y)))

in

next 1 1

let () =

let show (x, y, a) = Printf.printf "%3u! %c 1 = %u\n" x [|'-'; '+'|].(a) y in

factorials_plus_minus_one |> Seq.filter is_prime |> Seq.take 10 |> Seq.iter show</syntaxhighlight>

{{out}}

<pre>

1! + 1 = 2

2! + 1 = 3

3! - 1 = 5

3! + 1 = 7

4! - 1 = 23

6! - 1 = 719

7! - 1 = 5039

11! + 1 = 39916801

12! - 1 = 479001599

14! - 1 = 87178291199