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Idempotent semigroups and tropical algebraic sets

Zur IzhakianEugenii Shustin — 2012

Journal of the European Mathematical Society

The tropical semifield, i.e., the real numbers enhanced by the operations of addition and maximum, serves as a base of tropical mathematics. Addition is an abelian group operation, whereas the maximum defines an idempotent semigroup structure. We address the question of the geometry of idempotent semigroups, in particular, tropical algebraic sets carrying the structure of a commutative idempotent semigroup. We show that commutative idempotent semigroups are contractible, that systems of tropical...

Welschinger invariants of small non-toric Del Pezzo surfaces

Ilia ItenbergViatcheslav KharlamovEugenii Shustin — 2013

Journal of the European Mathematical Society

We give a recursive formula for purely real Welschinger invariants of the following real Del Pezzo surfaces: the projective plane blown up at q real and s 1 pairs of conjugate imaginary points, where q + 2 s 5 , and the real quadric blown up at s 1 pairs of conjugate imaginary points and having non-empty real part. The formula is similar to Vakil’s recursive formula [22] for Gromov–Witten invariants of these surfaces and generalizes our recursive formula [12] for purely real Welschinger invariants of real toric...

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