add problem 21

problems/21/solution.ex

@@ -0,0 +1,23 @@
+defmodule AmicableNumbers do
+  def divisors(n) do
+    [ 1 |
+    (for i <- 2..floor(:math.sqrt(n))//1,
+      div(n, i) == n / i do
+        [i, div(n, i)]
+    end |> Enum.flat_map(&(&1))) ]
+  end
+end
+
+ns = for i <- 1..10_000, into: %{} do
+  {i, AmicableNumbers.divisors(i) |> Enum.sum()}
+end
+
+for {a, b} <- ns,
+  a != b,
+  ns[b] == a,
+  reduce: 0 
+do
+  acc -> a + acc
+end
+|> IO.puts()
+