Do points of inflection have to be differentiable?

1 Answer
Sep 20, 2014

That is a good question! I had to revisit the definition in the Calculus book by Stewart, which states:

enter image source here

My answer to your question is no, a function does not need to be differentiable at a point of inflection; for example, the piecewise defined function

#f(x)={(x^2,if x<0), (sqrt{x},if x ge0):}#

is concave upward on #(-infty,0)# and concave downward on #(0,infty)# and is continuous at #x=0#, so #(0,0)# is an inflection point but not differentiable there.