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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 1990's 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.
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.
From this we can also define tropical exponentiation in the usual way as iterated tropical products.
A monomial of variables in this semiring is a linear map, represented in classical arithmetic as a linear function of the variables with integer coefficients. A polynomial in the semiring is the minimum of a finite number of such monomials, and is therefore a concave, continuous, 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:
- Tropical hypersurfaces are exactly the rational polyhedral complexes satisfying a "zero-tension" condition.
- Tropical surfaces are exactly the non-Archimedean amoebas over an algebraically closed non-archimedean field K.
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, ..., xn] 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[disambiguation needed], Bézout's theorem, the degree-genus formula, and the group law of the cubics all have tropical counterparts.
Tropical geometry was used by Economist Paul Klemperer to design auctions used by the Bank of England during the financial crisis in 2007. Shiozawa defined subtropical algebra as max-times or min-times semiring (instead of max-plus and min-plus). He found that Ricardian trade theory (international trade without input trade) can be interpreted as subtropical convex algebra.
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