This was a question asked to me in an exam which I couldn't answer:
Let $g:[0,1]\to \Bbb R$ be a continuous function, such that $0<g(x)<1$ for all $x\in[0,1]$. Let $f_n :[0,1] \to \Bbb R$ be a sequence of functions. Prove that if $f_n$ converges uniformly to $\mathit g$ then there exists $n_0 \in \Bbb N$ such that $0< f_n (x)< 1$ for all $n\geqslant n_0 ,\text{ for all $x \in [0,1]$}$.
What I tried to do was using the definition of uniform convergence, given $\varepsilon >0\ $there exists $n_0$ such that for all $n \geqslant n_0$, $|f_n – g|<\varepsilon$. This would mean that $-\varepsilon < f_n – g < \varepsilon$. Therefore, $$-\varepsilon + g < f_n < \varepsilon + g$$ and since $0<g<1$, $$-\varepsilon + 0< -\varepsilon + g< f_n < \varepsilon + g <\varepsilon + 1.$$ So, $-\varepsilon<f_n<\varepsilon +1$. I don't know how to go from here. Any help would be appreciated.
Best Answer
Let $M=\max g$ and let $m=\min G$. Then $0<m\leqslant M<1$. Take $\varepsilon>0$ such that, $\varepsilon<m$ and that $\varepsilon<1-M$. THere is a natural $N$ such that$$(\forall n\in\mathbb{N})(\forall x\in[0,1]):n\geqslant N\implies\bigl\lvert g(x)-f_n(x)\bigr\rvert<\varepsilon.$$But then, since$$(\forall x\in[0,1]):m\leqslant g(x)\leqslant M$$and since $\varepsilon<m$ and $\varepsilon<1-M$, we have$$(\forall x\in[0,1]):0<f_N(x)<1.$$