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. In this technique, which van Lint & Wilson (2001) call “one of the most important tools in combinatorics,” one describes a finite set X 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.
Famous quotes containing the words double and/or counting:
“You know, when these New Negroes have their convention—that is going to be the chairman of the Committee on Unending Agitation. Race, race, race!... Damn, even the N double A C P takes a holiday sometimes!”
—Lorraine Hansberry (1930–1965)
“Love is sinister,
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makes our thin bodies thinner.
This fellow Death
lacks mercy
and is good at counting our days.
And Master,
you, too, are subject
to the plague of jealousy
so think:
how could womenfolk,
soft as sprouts,
live like this?”
—Amaru (c. seventh century A.D.)