Problem 34: Find the sum of all numbers which are equal to the sum of the factorial of their digits.

The upper bound is 2540160, since (factorial 9)*7 = 2540160 and (factorial 9)^x grows faster than 10^x for x >= 7.

let problem34b =   
    {3..2540160}
    |> Seq.filter (fun i -> i = (i |> string |> Seq.map (string >> int) |> Seq.sumBy factorial))
    |> Seq.sum