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Viro’s method and T-curves. (English) Zbl 0879.14029
González-Vega, Laureano (ed.) et al., Algorithms in algebraic geometry and applications. Proceedings of the MEGA-94 conference, Santander, Spain, April 5-9, 1994. Basel: Birkhäuser. Prog. Math. 143, 177-192 (1996).
The paper presents some results obtained by means of the Viro method to construct plane real algebraic curves of a given degree with a prescribed oval arrangement. The construction discussed is of purely combinatorial nature: given a convex lattice polygon, its (lattice) triangulation, and a distribution of signs at the vertices of the triangulation, there can be constructed a broken line in the plane which is isotopic to a real non-singular curve defined by a polynomial with the given lattice polygon as its Newton polygon.
As application the author refers to his earlier construction of counterexamples to Ragsdale’s conjecture [I. Itenberg, C. R. Acad. Sci., Paris, Sér. I 317, No. 3, 277-282 (1993; Zbl 0787.14040)], and gives the complete classification of M-curves of degree $$4k+2$$ with one non-empty oval: for any positive integer $$p,n$$ satisfying $$p+n=2k(4k+1)+1$$ (the maximal possible number of ovals for the degree $$4k+2$$), $$p-n\equiv 1$$ mod 8 (Gudkov-Rokhlin congruence), $$\max\{p,n\}\leq 6k^2+3k+1$$ (Petrovsky inequalities), there exists a real curve of degree $$4k+2$$ with one non-empty oval, $$n$$ empty ovals inside the non-empty one, and $$p-1$$ empty ovals outside.
For the entire collection see [Zbl 0841.00016].
##### MSC:
 14P25 Topology of real algebraic varieties 52B20 Lattice polytopes in convex geometry (including relations with commutative algebra and algebraic geometry) 14Q05 Computational aspects of algebraic curves