diff --git a/DAS/Arithmetic.md b/DAS/Arithmetic.md
index fa656c1..7d56894 100644
--- a/DAS/Arithmetic.md
+++ b/DAS/Arithmetic.md
@@ -47,4 +47,9 @@ Only valid for GCD = 1
 	4. $1 = (1 + q_3 * q_2) * (A - q_1 * B) - q_3 * B$
 
 3. **Simplify to** $x * A + y * B$
-4. Coefficient of $A$ is the modular inverse
\ No newline at end of file
+4. Coefficient of $A$ is the modular inverse
+
+## Totient function
+Denoted by $phi(n) = n (1 - 1/p_1)(1 - 1/p_2)...(1-1/p_k)$ where $p_i$ is a prime factor of $n$.
+
+