In algebraic geometry, Mnev's universality theorem is a result which can be used to represent algebraic (or semi algebraic) varieties as realizations of oriented matroids, a notion of combinatorics.
Contents
Oriented matroids
For the purposes of Mnev's universality, an oriented matroid of a finite subset
The realization space of an oriented matroid is the space of all configurations of points
Stable equivalence of semialgebraic sets
For the purposes of Mnev's Universality, the stable equivalence of semialgebraic sets is defined as follows.
Let U, V be semialgebraic sets, obtained as a disconnected union of connected semialgebraic sets
We say that U and V are rationally equivalent if there exist homeomorphisms
Let
with
such that
The stable equivalence is an equivalence relation on semialgebraic subsets generated by stable projections and rational equivalence.
Mnev's Universality theorem
THEOREM (Mnev's universality theorem)
Let V be a semialgebraic subset in
History
Mnev's universality theorem was discovered by Nikolai Mnev in his Ph. D. thesis. It has numerous applications in algebraic geometry, due to Laurent Lafforgue, Ravi Vakil and others, allowing one to construct moduli spaces with arbitrarily bad behaviour.