Question

If function f is two times differentiable on R, its second derivative is scalar?

Answer 1

in fact, not on R, but on [a,b], \[0<a<b<\infty \]

Answer 2

Twice differentiable means that the second derivative exists, correct?

Answer 3

yes. But third derivative doesn't exist.

Answer 4

yeah and thats called the curvature