U.Seung's Log

set [ (999*999, 999, 999) ]

    |> Seq.unfold (fun pq ->

        let head = pq.MaximumElement

        let product, a, b = head

        let newElements =

            set [ ((a-1)*b, a-1, b); (a*(b-1), a, b-1) ]

                |> Set.filter (fun (_, a, b) -> a <= b)

        Some (product, ((pq.Remove head) + newElements)))

    |> Seq.filter is_parlindrome

    |> Seq.hd

    |> printfn "Problem #4 = %d"

let triangle_numbers =

    let rec triangle_number =

        function 0 -> 1 | n -> (n + 1 + triangle_number (n-1))

    Seq.init_infinite triangle_number

let triangle_numbers = Seq.unfold (fun (n, i) -> Some(n, (n+i, i+1))) (1, 2)

let triangle_numbers = Seq.init_infinite (fun n -> (n+2)*(n+1) / 2)

let factorize n =

    let rec factorize_horse n factor count result =

        if (n <= 1) then

            count::result

        else

            match (n % factor) with

            | 0 -> (factorize_horse (n/factor) factor (count+1) result)

            | _ -> (factorize_horse n (factor+1) 0 (count::result))          

    factorize_horse n 2 0 []

        |> List.filter (fun x -> x<>0)

let factor_count n =

    factorize n

        |> List.map (fun x -> x+1 )

        |> List.fold_left (*) 1

triangle_numbers

    |> Seq.filter (fun x -> 500 < (factor_count x))

    |> Seq.hd

    |> printfn "Problem #12 = %d"