From a32312fba0948e51cc3cf63ca27ce126231a699d Mon Sep 17 00:00:00 2001
From: Andrew Lorimer <andrew@lorimer.id.au>
Date: Sun, 28 Apr 2019 12:42:30 +1000
Subject: [PATCH] [methods] conditions for differentiation

---
 methods/calculus-ref.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/methods/calculus-ref.md b/methods/calculus-ref.md
index 5eb48c9..dab682a 100644
--- a/methods/calculus-ref.md
+++ b/methods/calculus-ref.md
@@ -42,6 +42,12 @@ A function is continuous if $L^-=L^+=f(x)$ for all values of $x$.
 
 $$f^\prime(x)=\lim_{h \rightarrow 0}{{f(x+h)-f(x)} \over h}$$
 
+Not differentiable at:
+
+- discontinuous points
+- sharp point/cusp
+- vertical tangents ($\infty$ gradient)
+
 ## Tangents & gradients
 
 **Tangent line** - defined by $y=mx+c$ where $m={dy \over dx}$  
-- 
2.43.2