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Weierstrass gap sequences at the ramification points of trigonal Riemann surfaces. (English) Zbl 0649.14009
A trigonal Riemann surface M of genus $$g\geq 5$$ is of n-th kind if $$\ell (nD)=n+1$$ and $$\ell (n+1)D\geq n+3$$, where D is the divisor of degree 3 associated to M. Total and ordinary ramification points of M as 3-sheeted covering of $${\mathbb{P}}^ 1$$ can be classified into two types [cf. M. Coppens, Indagationes Math. 47, 245-276 (1985; Zbl 0592.14025) and J. Pure Appl. Algebra 43, 11-25 (1986; Zbl 0616.14012)]. In this article the authors give a normalized defining equation of a trigonal Riemann surface M of n-th kind, and by investigating it they determine the types of ramification points of M and show the existence of Riemann surfaces with various types of ramification points. Some estimates on the numbers of ramification points are also obtained.
Reviewer: R.Horiuchi

##### MSC:
 14H55 Riemann surfaces; Weierstrass points; gap sequences 14H20 Singularities of curves, local rings 30F10 Compact Riemann surfaces and uniformization 14H30 Coverings of curves, fundamental group
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##### References:
 [1] Coolidge, J.L., A treatise on algebraic plane curve, (1959), Dover New York [2] Coppens, M., The Weierstrass gap sequences of the total ramification points of trigonal covering of P1, Indag. math., 47, 245-270, (1985) · Zbl 0592.14025 [3] Coppens, M., The Weierstrass gap sequence of the ordinary ramification points of trigonal coverings of P1, Existence of a kind of Weierstrass gap sequence, J. pure appl. algebra, 43, 11-25, (1986) · Zbl 0616.14012 [4] Kato, T., On Weierstrass points whose first non-gaps are three, J. reine angew. math., 316, 99-109, (1980) · Zbl 0419.30037
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