diff --git a/src/main/java/me/hatter/math/FibonacciSequence.java b/src/main/java/me/hatter/math/FibonacciSequence.java
index d328766..93e6b8e 100644
--- a/src/main/java/me/hatter/math/FibonacciSequence.java
+++ b/src/main/java/me/hatter/math/FibonacciSequence.java
@@ -8,6 +8,7 @@ import java.math.RoundingMode;
 import java.util.ArrayList;
 import java.util.List;
 
+// https://oi-wiki.org/math/combinatorics/fibonacci/
 public class FibonacciSequence {
     static final BigDecimal SQRT5 = MathUtil.sqrt(BigDecimal.valueOf(5), 100);