diff --git a/day09.ua b/day09.ua
new file mode 100644
index 0000000..61b586f
--- /dev/null
+++ b/day09.ua
@@ -0,0 +1,16 @@
+$ 0 3 6 9 12 15
+$ 1 3 6 10 15 21
+$ 10 13 16 21 30 45
+TestInput ←
+
+Parse ← ⊜(⊜⋕≠@ .)≠@\n.
+
+Differences ← ⍢(⊂:□≡/-◫2°□⊢⇌.)(¬≍⊚⧻.°□⊢⇌)¤□
+NextElement ← /+≡(⊢⇌)Differences
+PrevElement ← /-⇌≡⊢Differences
+
+/+≡NextElement Parse TestInput
+/+≡PrevElement Parse TestInput
+
+/+≡NextElement Parse &fras "day09.in"
+/+≡PrevElement Parse &fras "day09.in"