×

The space of lines in cyclic covers of projective space. (English) Zbl 1422.14054

Let \(W\) be a smooth complex projective variety of dimension \(n\), endowed with an ample line bundle \(h\). A line of \((W, h)\) is a rational curve in \(W\) of \(h\)-degree \(1\). Let \(F(W)\) be the space of lines of \((W, h)\). If non-empty, its dimension is \(\dim(F(W)) \leq\exp\dim (F(W))= (-K_W) \cdot \ell + n - 3\), where \(\ell\) is a line of \((W, h)\), and it is of interest to know when this is an equality. This happens e.g., for a general hypersurface \(W \subset \mathbb P^{n+1}\) of degree \(d > 2n-1\), with \(h\) being the hyperplane bundle. In the paper under review, the authors consider the space of lines in cyclic covers of projective spaces and prove the following result. Let \(m, n, d\) be positive integers such that \(md > 2n-3\) and \(k := 2(n-1)-d(m-1) \geq 0\), let \(w : Y \to \mathbb P^n\) be a cyclic cover of degree \(m\), branched along a general hypersurface \(X \subset \mathbb P^n\) of degree \(md\) and \(h = w^*\mathcal O_{\mathbb P^n}(1)\). Then \(F(Y)\) is smooth of dimension \(k\) and irreducible if \(k \geq 1\); in particular, \(F(Y)\) has the expected dimension. For \(n = 3, m = d = 2\), this was already proven by [A. S. Tikhomirov [Izv. Akad. Nauk SSSR, Ser. Mat. 44, 415–442 (1980; Zbl 0434.14023)]. The result is proven by connecting the lines of \((Y, h)\) to the \(m\)-contact order lines in \(X\), i.e., lines \(\ell \subset \mathbb P^n\) whose local intersection number with \(X\) at each point of \(\ell \cap X\) is a multiple of \(m\). Actually, the authors study the subvariety \(S_m(X)\) of the Grassmannian \(G\) of lines of \(\mathbb P^n\) parameterizing such lines, proving that it is smooth of dimension \(k\) and show that \(F(Y)\) is an unramified cover of \(S_m(X)\) of degree \(m\). Furthermore, when \(k = 0\), they obtain an explicit enumerative formula expressing the number of \(m\)-contact order lines of \(X\) in terms of intersection numbers of Schubert cycles on \(G\).

MSC:

14N05 Projective techniques in algebraic geometry
14E20 Coverings in algebraic geometry
14J45 Fano varieties

Citations:

Zbl 0434.14023
Full Text: DOI

References:

[1] Fulton, W., Intersection Theory (1998), Springer-Verlag: Springer-Verlag Berlin · Zbl 0885.14002
[2] Hartshorne, R., Algebraic Geometry, Graduate Texts in Mathematics, vol. 52 (1977), Springer-Verlag: Springer-Verlag New York, Heidelberg · Zbl 0367.14001
[3] Hwang, J.-M.; Kim, H., Varieties of minimal rational tangents on double covers of projective space, Math. Z., 275, 1-2, 109-125 (2013) · Zbl 1282.14070
[4] Kollár, J., Rational Curves on Algebraic Varieties, Ergebnisse der Mathematik und ihrer Grenzgebiete, vol. 32 (1996), Springer
[5] Pragacz, P., Miscellany on the zero schemes of sections of vector bundles, (Algebraic Cycles, Sheaves, Shtukas, and Moduli. Algebraic Cycles, Sheaves, Shtukas, and Moduli, Trends in Mathematics (2007), Birkhäuser: Birkhäuser Basel), 105-116 · Zbl 1142.14315
[6] Tihomirov, A. S., Geometry of the Fano surface of a double \(P^3\) branched in a quartic, Izv. Akad. Nauk SSSR, Ser. Mat., 44, 2, 415-442 (1980), 479 (in Russian) · Zbl 0434.14023
This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. In some cases that data have been complemented/enhanced by data from zbMATH Open. This attempts to reflect the references listed in the original paper as accurately as possible without claiming completeness or a perfect matching.