proof

(a) Show that the formula $$\left[ {\left( { \sim p \vee Q} \right) \Rightarrow \left( {q \Rightarrow p} \right)} \right]$$ is not a tautology. (b) Let $$A$$ be a tautology and $$B...

Let (A, *) be a semigroup. Furthermore, for every a and b in A, if $$a\, \ne \,b$$, then $$a\,*\,b \ne \,\,b\,*\,a$$. (a) Show that for every a in A a * a = a (b) Show that for eve...

Prove that in a finite graph, the number of vertices of odd degree is always even.