Bound and free variables
When a quantifier is used on the variable x or when we assign
a value to this variable, we say that this occurrence of the variable is
An occurrence of a variable that is not bound is said to be free.
--- x is bound, y is free
--- the first occurrence of x is bound; y
and the second occurrence of x are free
Turn the propositional function x•y = 1 into a true proposition by
binding its variables, assuming that the universe of discourse is
the set of positive real numbers.