initial commit

12.fs

@@ -0,0 +1,17 @@
+module _12
+open System
+
+let fiveDivisorTriangle =
+    let root (n : int) = int( Math.Sqrt(float(n)) )
+    // observation: if number has a factor < sqrt(num), then it has a corresponding
+    //   factor > sqrt(num)
+    let factorCount x = Seq.sum (seq { for i = 1 to root x do if x % i = 0 then yield 2 })
+
+    let result =
+        (1, 0) |> Seq.unfold (fun (num,acc) -> 
+                    Some(num + acc, (num + 1, num + acc)))
+                 |> Seq.filter (fun x -> 
+                    let factCount = factorCount x
+                    factCount >= 500 )
+                 |> Seq.head
+    result