Problem 48: Find the last ten digits of the series, 1^1 + 2^2 + 3^3 + ... + 1000^1000.

Again, easy problem using functional composition, the “hard” work was done in our previous implementation of the Digits module. The answer was produced surprisingly fast. I would like a better solution to finding the last 10 digits than to convert the entire sequence to an array.

let problem48a =
    let digits =
        {1..1000}
        |> Seq.sumBy (fun n -> BigInteger.Pow(BigInteger(n), n))
        |> Digits.fromBigInt
        |> Seq.toArray

    Array.sub digits (digits.Length - 10) 10
    |> Digits.toInt64 //just makes it easier to copy and paste result