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?
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.