#### Problem

We shall say that an n-digit number is pandigital if it makes use of all the digits 1 to n exactly once. For example, 2143 is a 4-digit pandigital and is also prime.

What is the largest n-digit pandigital prime that exists?

#### Solution

let rec distribute e = function | [] -> [[e]] | x::xs' as xs -> (e::xs)::[for xs in distribute e xs' -> x::xs] let rec permute = function | [] -> [[]] | e::xs -> List.collect (distribute e) (permute xs) let hasDivisor(n) = let upperBound = int64(sqrt(double(n))) [2L..upperBound] |> Seq.exists (fun x -> n % x = 0L) let isPrime(n) = if n = 1L then false else not(hasDivisor(n)) let answer = [1..9] |> List.collect (fun m -> [1..m] |> permute) |> List.map (fun l -> l |> List.map string |> List.reduce (fun acc item -> acc + item)) |> List.map int64 |> List.filter isPrime |> List.max

This is a simple brute force solution which for n = 1..9 generates all permutations of 1..n, for each permutation the digits are merged into a number, e.g. [1; 4; 8; 7] –> 1487, and the max prime number is returned.

Use this method to check for primes, is 10 times faster.

let isPrime n =

if n float |> sqrt |> int

let rec check i = i > maximumDivisor || (n % i 0 && check (i + 1))

check 2

Check out some of the later answers, I started to use a siev which was able to generate 1 million primes in well under a second on my machine. (didn’t know what a siev is when I put this answer together unfortunately…)