# Infinite set, disjunction, and tautology

1. Apr 18, 2010

### iambored

Let {A1, A2, A3, ... } be an in finite set of formulas in propositional logic. Assume that
for every valuation v there is some n (depending on v) such that v(An) = 1. Show
then that there is some fixed m with A1 $$\vee$$ A2 $$\vee$$ ... $$\vee$$ Am a tautology.

This is equivalent to showing that v(Ai) = 1 for at least one 1$$\leq$$i$$\leq$$m. But I'm not sure where to proceed from here.

2. Apr 19, 2010

### JSuarez

Start by considering the set $\left\{A_1,A_1\vee A_2,A_1\vee A_2 \vee A_3\cdots\right\}$ and assume that none of its elements is a tautology. Note also that any valuation may be identified with an infinite binary sequence.