diff --git a/journal/2023-03-21.md b/journal/2023-03-21.md
index 520742282..d5b65e582 100644
--- a/journal/2023-03-21.md
+++ b/journal/2023-03-21.md
@@ -9,3 +9,4 @@
   - [[sila]]
     - [ ] answer ping by [[sl]] 
     - [ ] answer ping by [[hd]]
+- [[perturbation theory]] https://en.wikipedia.org/wiki/Perturbation_theory