diff --git a/documents/by-course/pstat-120a/course-notes/main.typ b/documents/by-course/pstat-120a/course-notes/main.typ
index 7469228..e84d6e9 100644
--- a/documents/by-course/pstat-120a/course-notes/main.typ
+++ b/documents/by-course/pstat-120a/course-notes/main.typ
@@ -1301,7 +1301,7 @@ $
 #definition[
   For an integer valued random variable $X$, we say $X ~ "Poisson"(lambda)$ if it has p.m.f.
 
-  $ P(X = k) = e^(lambda) lambda^k / k! $
+  $ P(X = k) = e^(-lambda) lambda^k / k! $
 
   for $k in {0,1,2,...}$ for $lambda > 0$ and