Tropical geometry

Tropical geometry is a relatively new area in mathematics, which might loosely be described as a piece-wise linear or skeletonized version of algebraic geometry. Its leading ideas had appeared in different guises in previous works of George M. Bergman and of Robert Bieri and John Groves, but only since the late nineties has an effort been made to consolidate the basic definitions of the theory. This effort has been in great part motivated by the strong applications to enumerative algebraic geometry uncovered by Grigory Mikhalkin.

The adjective tropical is given in honor of the Brazilian mathematician Imre Simon, who pioneered the field.

Basic definitions

We will use the min convention, that tropical addition is classical minimum. It is also possible to cast the whole subject in terms of the max convention, negating throughout, and several authors make this choice.

Consider the tropical semiring (also known as the min-plus algebra due to the definition of the semiring). This semiring, (R ∪ {∞}, ⊕, ⊗), is defined with the operations as follows:

$x \oplus y = \min\{\, x, y \,\},\,$

$x \otimes y = x + y.\,$

A monomial in this semiring is a linear map, and a polynomial is the minimum of a finite number of such functions, and therefore a concave, piecewise linear function.

The set of points where a tropical polynomial F is non-differentiable is called its associated tropical hypersurface.

There are two important characterizations of these objects:

1. Tropical hypersurfaces are exactly the rational polyhedral complexes satisfying a "zero-tension" condition.
2. Tropical surfaces are exactly the non-Archimedean amoebas over an algebraically closed field K with a non-Archimedean valuation.

These two characterizations provide a "dictionary" between combinatorics and algebra. Such a dictionary can be used to take an algebraic problem and solve its easier combinatorial counterpart instead.

The tropical hypersurface can be generalized to a tropical variety by taking the non-archimedean amoeba of ideals I in K[x₁, ..., x_n] instead of polynomials. It has been proved that the tropical variety of an ideal I equals the intersection of the tropical hypersurfaces associated to every polynomial in I. This intersection can be chosen to be finite.

There are a number of articles and surveys on tropical geometry. The study of tropical curves (tropical hypersurfaces in R²) is particularly well developed. In fact, for this setting, mathematicians have established analogues of many classical theorems; e.g., Pappus's theorem, Bézout's theorem, the degree-genus formula, and the group law of the cubics all have tropical counterparts.

See also

• Max-plus algebra

External links

Introductory articles and surveys

• First Steps in Tropical Geometry
• Tropical geometry of statistical models
• The Tropical Grassmanian
• Enumerative tropical algebraic geometry in R2
• Amoebas of algebraic varieties and tropical geometry
• Tropical Mathematics
• Non-archimedean amoebas and tropical varieties
• Computing Tropical Varieties
• Tropical Geometry and its applications
• Tropical algebraic geometry

Talk on tropical geometry

• Tropical Geometry, I