Editing Double counting (proof technique) (section)

{{Short description|Type of proof technique}}
In [[combinatorics]], '''double counting''', also called '''counting in two ways''', is a [[combinatorial proof]] technique for showing that two expressions are equal by demonstrating that they are two ways of counting the size of one [[Set (mathematics)|set]]. In this technique, which {{harvtxt|van Lint|Wilson|2001}} call "one of the most important tools in combinatorics",{{sfn|van Lint|Wilson|2001}} one describes a [[finite set]] from two perspectives leading to two distinct expressions for the size of the set. Since both expressions equal the size of the same set, they equal each other.