If for any a, limh→0f(a+h)=f(a), then limh→0DELTA(f) / h exists
Nope.
Proof for differentiability:
If for some a, \(\displaystyle\lim_{h\rightarrow 0}\dfrac{f(a+h)-f(a)}{h}\) exists, then f(x) is differentiable at the Cartesian point (a, f(a))
