Let $X$ be some subset of euclidean space and for the bounded function $f : X \to \mathbb{R}$ and let $f(x) = \sup f(X)$. Is the limit
$\lim_{t \to 0} \frac{f(x+ta) - f(x)}{t}$
if it exists guaranteed to be nonpositive?
Let $X$ be some subset of euclidean space and for the bounded function $f : X \to \mathbb{R}$ and let $f(x) = \sup f(X)$. Is the limit
$\lim_{t \to 0} \frac{f(x+ta) - f(x)}{t}$
if it exists guaranteed to be nonpositive?
No. Let $X=[0,1]$ and $f(x)=x$. Then $f(1)=\sup f(X)$, and
$ \lim_{t\to 0}\frac{f(x+ta)-f(x)}t=\frac{x+ta-x}t=a\;, $
which can be positive, zero or negative depending on $a$.
By the way, I think one question mark would have been enough.