Mnëv's universality theorem

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In algebraic geometry, Mnëv'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.[1][2][3]

Oriented matroids[edit]

For the purposes of Mnëv's universality, an oriented matroid of a finite subset is a list of all partitions of points in S induced by hyperplanes in . In particular, the structure of oriented matroid contains full information on the incidence relations in S, inducing on S a matroid structure.

The realization space of an oriented matroid is the space of all configurations of points inducing the same oriented matroid structure on S.

Stable equivalence of semialgebraic sets[edit]

For the purposes of Mnëv'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 defined by rational maps.

Let be semialgebraic sets,


with mapping to under the natural projection deleting last d coordinates. We say that is a stable projection if there exist integer polynomial maps

such that

The stable equivalence is an equivalence relation on semialgebraic subsets generated by stable projections and rational equivalence.

Mnëv's universality theorem[edit]

THEOREM (Mnëv's universality theorem)

Let V be a semialgebraic subset in defined over integers. Then V is stably equivalent to a realization space of a certain oriented matroid.


Mnëv's universality theorem was discovered by Nikolai Mnëv in his 1986 Ph.D. thesis.[4] 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.


  • Universality Theorem, a lecture of Nikolai Mnëv (in Russian).
  • Nikolai E. Mnëv, The universality theorems on the classification problem of configuration varieties and convex polytopes varieties (pp. 527–543), in "Topology and geometry: Rohlin Seminar." Edited by O. Ya. Viro. Lecture Notes in Mathematics, 1346. Springer-Verlag, Berlin, 1988.
  • Vakil, Ravi (2006), "Murphy's Law in algebraic geometry: Badly-behaved deformation spaces", Inventiones Mathematicae, 164: 569–590, arXiv:math/0411469.
  • Richter-Gebert, Jürgen (1995), "Mnëv's Universality Theorem Revisited", Séminaire Lotharingien de Combinatoire, B34h: 15


  1. ^ Mnev, N. E. (1988), "The universality theorems on the classification problem of configuration varieties and convex polytopes varieties", Lecture Notes in Mathematics, Springer Berlin Heidelberg, pp. 527–543, ISBN 9783540502371, retrieved 2018-09-18
  2. ^ Sturmfels, Bernd; Gritzmann, Peter, eds. (1991-06-26). Applied Geometry and Discrete Mathematics: The Victor Klee Festschrift. DIMACS Series in Discrete Mathematics and Theoretical Computer Science. Providence, Rhode Island: American Mathematical Society. ISBN 9780821865934.
  3. ^ Vershik, A. M. (1988), "Topology of the convex polytopes' manifolds, the manifold of the projective configurations of a given combinatorial type and representations of lattices", Lecture Notes in Mathematics, Springer Berlin Heidelberg, pp. 557–581, ISBN 9783540502371, retrieved 2018-09-18
  4. ^ "Nikolai Mnev Homepage". Retrieved 2018-09-18.