$ f:\Omega \rightarrow \Re ^ m, \Omega \subset \Re ^n $
$ f $ is said to be k-th continuously differentiable on $ \Omega $, $ f \in C^{k} $,
if each component of f has continuous partials of order k on $ \Omega $.
Example.
- For k=0, f is said to be coutinuous
- For k=1, f is said to be continuously differentiable
