1 files changed, 24 insertions, 0 deletions
diff --git a/21.lua b/21.lua
new file mode 100644
index 0000000..d4a5334
--- /dev/null
+++ b/21.lua
@@ -0,0 +1,24 @@
+function d(n)
+    if n < 2 then return 0 end
+    local ret = 1
+    local test = 2
+    local limit = math.sqrt(n)
+    while test < limit do
+        if n % test == 0 then
+            ret = ret + test + n / test
+        end
+        test = test + 1
+    end
+    if limit == math.floor(limit) then ret = ret + limit end
+    return ret
+end
+
+local sum = 0
+for i = 1, 9999 do
+    local di = d(i)
+    if d(di) == i and i ~= di then
+        print(i .. ": " .. di)
+        sum = sum + i
+    end
+end
+print(sum)