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Let $X$ be a Tychonoff topological space and let $x\in X$. Let $B\subset C(X)$ be convex and compact in the topology of pointwise convergence, and such that $f(x)=1$, for every $f\in B$.

Is there an open neighborhood $U$ of $x$ such that $f(y)\ne 0$, for every $f\in B$ and $y\in U$?

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  • $\begingroup$ Without convexity a counterexample would be $X=\mathbb R$ and $B=\{f_n:n\in\mathbb N\} \cup\{1\}$ where $f_n$ are $1$ outside $(0,2/n)$, $f_n(1/n)=0$ and affine linear and on the two halfs of $[0,2/n]$ so that they are continuous. Then $f_n\to 1$ pointwise which implies compactness. $\endgroup$ Commented Aug 11, 2020 at 16:11
  • $\begingroup$ @JochenWengenroth yeah, perhaps I should have put this example in the question itself. Luckily (?), in my situation I have additional convexity assumption $\endgroup$
    – erz
    Commented Aug 11, 2020 at 16:41

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Since $C(X)$ is not complete one cannot take the closed convex hull of the example in the comment. But what about this:

Let $g_n=1-f_n$ with $f_n$ as in my comment. Since the $g_n$ are bounded by one, the linear map $T:\ell^1\to C(X)$, $\lambda\mapsto \sum\limits_{n=1}^\infty \lambda_ng_n$ is well defined. We would like to have $T$ continuous as a map $(\ell^1,\sigma(\ell^1,c_0)) \to (C(X),pw)$ (where $c_0$ is the space of all null sequences so that $\ell^1$ is its dual). This follows from the pointwise convergence to $0$ of all sequences $(g_n(x))_{n\in\mathbb N}$. The unit ball $K$ of $\ell^1$ is weak$^*$ compact à la Alaoglu and hence $T(K)$ is compact and convex in $(C(X),pw)$. Then $B=\{1-g: g\in T(K)\}$ is convex and compact in the pointwise topology, it satisfies $f(0)=1$ for all $f\in B$ (because $g_n(0)=0$ for all $n$), and it contains all $f_n=1-g_n=1-T(e_n)$ with the standard unit vectors $e_n$ of $\ell^1$.

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