In graph theory, the **planar separator theorem** is a form of isoperimetric inequality for planar graphs, that states that any planar graph can be split into smaller pieces by removing a small number of vertices. Specifically, the removal of O(√*n*) vertices from an *n*-vertex graph (where the *O* invokes big O notation) can partition the graph into disjoint subgraphs each of which has at most 2*n*/3 vertices.

A weaker form of the separator theorem with O(√*n* log *n*) vertices in the separator instead of O(√*n*) was originally proven by Ungar (1951), and the form with the tight asymptotic bound on the separator size was first proven by Lipton & Tarjan (1979). Since their work, the separator theorem has been reproven in several different ways, the constant in the O(√*n*) term of the theorem has been improved, and it has been extended to certain classes of nonplanar graphs.

Repeated application of the separator theorem produces a separator hierarchy which may take the form of either a tree decomposition or a branch-decomposition of the graph. Separator hierarchies may be used to devise efficient divide and conquer algorithms for planar graphs, and dynamic programming on these hierarchies can be used to devise exponential time and fixed-parameter tractable algorithms for solving NP-hard optimization problems on these graphs. Separator hierarchies may also be used in nested dissection, an efficient variant of Gaussian elimination for solving sparse systems of linear equations arising from finite element methods.

Read more about Planar Separator Theorem: Statement of The Theorem, Example, Lower Bounds, Separator Hierarchies, Other Classes of Graphs, See Also

### Famous quotes containing the word theorem:

“To insure the adoration of a *theorem* for any length of time, faith is not enough, a police force is needed as well.”

—Albert Camus (1913–1960)