Pencil (mathematics)

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This article is about a class of geometric objects. For other uses, see Pencil (disambiguation).

A pencil is a family of geometric objects, such as lines, that have a common property, such as passage through a given line in a given plane.

In more technical language, a pencil is the special case of a linear system of divisors in which the parameter space is a projective line. Typical pencils of curves in the projective plane, for example, are written as

λC + μC′ = 0

where

C = 0, C′ = 0

are plane curves.

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