A completely regular space X is an F-space if for each functionally open set $M\subset X$ every continous function $f: M \rightarrow I $ continuously extendable over X.
I want to prove that a completely regular space X is an F-space iff any two disjoint sets functionally open in X are completely separated. $(\leftarrow :)$ It is known that any disjoint functionally closed sets A,B in X are completelty separated.Hence, every disjoint functionally open sets are completely separated. So, one side is clear. $(:\rightarrow )$ For the other side , lets take a functionally open set M and take a continuous function $f:M\rightarrow I$. I need a two disjoint functionally open set in X. One of them is M. For the other, I take $N={f^{-1}}(0)$ is functionally closed and complement of N in X is functionally open. I say $N^{c}\cap M=\emptyset$. If it is true, then I can use the completely separated property. then there exists a continous function $h:X\rightarrow I$ such that $h(x)=0$ for each $x\in M$ and $h(x)=1$for each $x\in N^{c}$ Hence, can we say h is the extension of f over X?