[methods] conditions for differentiation

diff --git a/methods/calculus-ref.md b/methods/calculus-ref.md
index 5eb48c9bbbaa77715b281487cf1274a9d2fa6dec..dab682aeeb3c27c7666e0e0f8a103ea7d1e47bbd 100644 (file)
--- 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 $m={dy \over dx}$