Definition:Free Occurrence

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Let $\LL_1$ be the language of predicate logic.

Let $\mathbf A$ be a WFF of $\LL_1$.

An occurrence of a variable $x$ in $\mathbf A$ is said to be a free occurrence if and only if it is not bound.

Also known as

Some authors gloss over the difference between:

a free variable: a variable which exists in a WFF only as free occurrences


a free occurrence of a variable which may otherwise exist as a bound variable.

Also see