initial commit

21.fs

@@ -0,0 +1,19 @@
+module _21
+
+let amicalbleNumbers len =
+    // get divisor sums
+    let dSums = common.divisors len |> Array.map (List.fold (+) 0)
+    // find amicable ones
+    [2..len] |> List.filter (fun i -> 
+        dSums.[i] > 0 &&
+        dSums.[i] <= len && 
+        not (i = dSums.[i]) &&
+        i < dSums.[i] &&
+        i = dSums.[dSums.[i]]
+        )
+        |> List.map (fun i -> (i, dSums.[i]))
+
+let amicalbleSum =
+    let a = amicalbleNumbers 10000
+    let all = List.append (a |> List.map fst) (a |> List.map snd)
+    all |> List.sum