diff --git a/units/en/unit2/4.mdx b/units/en/unit2/4.mdx
index e147aee..3ad4d49 100644
--- a/units/en/unit2/4.mdx
+++ b/units/en/unit2/4.mdx
@@ -150,7 +150,7 @@ For a full list of references, check out the [tutorial](https://huggingface.co/s
 - **Feedback Systems: An Introduction for Scientists and Engineers** (2008)  
   Karl Johan Åström and Richard M. Murray  
   A comprehensive introduction to feedback control systems, covering the principles that underlie closed-loop control in robotics.  
-  [Book Website](http://www.cds.caltech.edu/~murray/amwiki/index.php/Main_Page)
+  [Book Website](https://www.fbswiki.org/wiki/index.php/Feedback_Systems:_An_Introduction_for_Scientists_and_Engineers)
 
 - **Real-Time Obstacle Avoidance for Manipulators and Mobile Robots** (1986)  
   Oussama Khatib