diff --git a/tools/int_seq.py b/tools/int_seq.py
index ee701a7..92b5250 100644
--- a/tools/int_seq.py
+++ b/tools/int_seq.py
@@ -28,3 +28,11 @@ def triangular(n: int) -> int:
     0, 1, 3, 6, 10, 15, ...
     """
     return n * (n + 1) // 2
+
+
+def pentagonal(n: int) -> int:
+    """
+    A pentagonal number is a figurate number that extends the concept of triangular and square numbers to the pentagon
+    0, 1, 5, 12, 22, 35, ...
+    """
+    return ((3 * n * n) - n) / 2