Lambda calculus
Free variables

The free variables of a term are those variables not bound by an abstraction. The set of free variables of an expression is defined inductively:

For example, the lambda term representing the identity λ x. x {\displaystyle \lambda x.x} has no free variables, but the function λ x. y {\displaystyle \lambda x.y} x {\displaystyle x} has a single free variable, y {\displaystyle y}.
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