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Any occurrence a variable in a formulate not in the scope of a quantifier is said to be a free occurrence
Otherwise it is called a bound occurrence
Thus, if x is a free variable in φ it is bound in ∀xφ and ∃xφ
A formula with no free variables is called a closed formula
Example: x and y are bound variables, z is a free variable
∀x∀y(P(f(x)) → ¬(P(x)) → Q(f(y), x, z)))
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