diff --git a/lectures/sympy.md b/lectures/sympy.md
index 4210369..fd76921 100644
--- a/lectures/sympy.md
+++ b/lectures/sympy.md
@@ -41,6 +41,7 @@ translation:
 </div>
 ```
 
+(sympy)=
 # {index}`SymPy <single: SymPy>`
 
 ```{index} single: Python; SymPy