nLab tropical semiring

Definitions

The tropical semiring is a semiring $(\mathbb{R}\cup \{\infty\},\oplus,\otimes)$ with addition $x\oplus y = min(x,y)$ and multiplication $x\otimes y = x+y$. (This is semiring in the sense of rig, hence sometimes the tropical rig.)

Tropical geometry is often thought as the algebraic geometry over the tropical semiring.

Terminology

The tropical rig is also called the min-plus algebra. There is a related, in fact isomorphic rig called the max-plus algebra. (Some authors use the term ‘tropical algebra’ for the max-plus rather than the min-plus algebra. The theories, of course, run in parallel, as each is the negative of the other.)

In his survey article, cited below, Pin uses the term for a wide range of similar idempotent semirings. For instance $\mathcal{M} = (\mathbb{N} \cup{\infty}, min, +)$ is a tropical semiring introduced by Imre Simon in 1978.

Properties

• The tropical semiring is an example of an idempotent semiring, since for all elements $x$, we have $x\oplus x=x$.

Examples

In the min-plus algebra we have, for instance:

$$\begin{aligned} (5\oplus 6)\otimes 7 & = min(5,6) + 7 \\ & = 5 + 7 \\ & = 12 \,. \end{aligned}$$

Applications

Apart from applications in tropical geometry, the min-plus and max-plus algebras have extensive use in providing algebraic models for simple discrete event systems related to timed Petri nets.

References

• The use of the tropical algebra in discrete event systems is handled in many sources. A slightly old set of notes (in French) for an introductory course by Stéphane Gaubert, of INRIA Saclay. They can be found here.

• Jean-Eric Pin, Tropical semirings, Hal preprint, hal-00113779, and in the next reference, pp.50-69, 1998, Publ. Newton Inst. 11.

Book collection of articles on idempotent semirings:

• J. Gunawardena (ed.), Idempotency, Cambridge University Press, 2001,

An original source:

• Imre Simon, (1978), Limited Subsets of a Free Monoid, in Proc. 19th Annual Symposium on Foundations of Computer Science, Piscataway, N.J., Institute of Electrical and Electronics Engineers, 143–150 (doi:10.1109/SFCS.1978.21)

See also:

• Wikipedia, Tropical semiring