diff --git a/stochastic-branch-takeoff.md b/stochastic-branch-takeoff.md
index 3412f74..52933ca 100644
--- a/stochastic-branch-takeoff.md
+++ b/stochastic-branch-takeoff.md
@@ -50,6 +50,8 @@ Something like this:
 
 ![[function-example.excalidraw.md]]
 
+$f'(x)$ is a **sigmoid** function.
+
 This is preferable to a floor-ceiling piece-wise function
 since it remains differentiable.