diff --git a/maths/fermat_little_theorem.py b/maths/fermat_little_theorem.py
index 4a3ecd05ce91..76d111f6800d 100644
--- a/maths/fermat_little_theorem.py
+++ b/maths/fermat_little_theorem.py
@@ -6,6 +6,16 @@
 
 
 def binary_exponentiation(a: int, n: float, mod: int) -> int:
+    """
+    Compute (a^n) % mod using binary exponentiation.
+
+    >>> binary_exponentiation(2, 5, 13)
+    6
+    >>> binary_exponentiation(3, 4, 5)
+    1
+    >>> binary_exponentiation(10, 0, 7)
+    1
+    """
     if n == 0:
         return 1