Editing Lambda calculus (section)

==== Free variables ====
The ''free variables''{{efn|free variables in lambda Notation and its Calculus are comparable to [[Free variables and bound variables|linear algebra and mathematical concepts of the same name]]}} of a term are those variables not bound by an abstraction. The set of free variables of an expression is defined inductively:
* The free variables of <math>x</math> are just <math>x</math>
* The [[Set theory|set]] of free variables of <math>\lambda x.t</math> is the set of free variables of <math>t</math>, but with <math>x</math> removed
* The [[Set theory|set]] of free variables of <math>t s</math> is the union of the set of free variables of <math>t</math> and the set of free variables of <math>s</math>.

For example, the lambda term representing the identity <math>\lambda x.x</math> has no free variables, but the function <math>\lambda x. y x</math> has a single free variable, <math>y</math>.