Since, no matter which values we choose for p and q, it's always the case that one of these four parts yields false. It's a conjunction of four parts, and this formula is unsatisfiable. So, the conjunction of the three parts also yields true. Why? Well, if we choose p to be true and q to be false, we see that all parts of the formula yield true. Here we have a formula p or q, and not p or not q, and p or not q. The mapping of the variables to these values is such a way that the formula yields true, that mappings is called the satisfying assignment. The propositional formula is defined to be satisfiable, abbreviated to SAT if it is possible to give values to the variables in such a way that the formula yields true. These operators are the negation, the "not", the disjunction, the "or", the conjunction, the "and", the implication, written as an arrow, and p implies q is equivalent to not p or q, and the bi-implication which, as the name already suggests, is just a combination of two implications. A propositional formula is by definition composed from Boolean variables and the number of operations. I will give the definition, and I will make some basic observations.
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