[cats...]

we know that the optimal bluffing frequency is s.t the odds against bluffing are the same as the pot odds the opponent receives. At the optimal bluffing frequency it doesn't matter if the opponent always calls or always folds. But suppose the opponent wavers somewhere in between (i.e sometimes calls, sometimes folds). Will he not be playing optimally? how does 1 prove that the opponents optimal strategy is to always call or always fold?

thanks.