Tangents and Secants of Algebraic Varieties by F. L. Zak

By F. L. Zak

Over the last 20 years algebraic geometry has skilled a extraordinary shift from improvement of summary theories to research of concrete houses of projective types. Many difficulties of classical algebraic geometry based on linear platforms, projections, embedded tangent areas, etc. Use of contemporary ideas has made it attainable to make growth on a few of these difficulties. Following those topics, this publication covers those subject matters, between others: tangent areas to subvarieties of projective areas and complicated tori, projections of algebraic forms, category of Severi forms, greater secant types, and type of Scorza types over an algebraically closed box of attribute 0.

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Summing up, we obtain the following result. 8. Theorem. If a variety X n ⊂ PN corresponds to the orbit of highest weight vector of an irreducible representation of a simple Lie group G and dim SX < N ≤ 2n + 1, then X is one of the seven varieties A1 )–A4 ), C), E), F). The varieties A1 ), A3 ), and E) are Severi varieties, and the varieties A0 ), A2 ), C), and F) are hyperplane sections of Severi varieties. 8 we obtain the following result. 9. Theorem. Over an algebraically closed field K of characteristic zero there exist exactly four Severi varieties corresponding to orbits of linear actions of algebraic groups, viz.

E. codimPN X n = N − n ≥ 2N ≥ 2s + 2 ≥ 2n + r − b + 2, 1 (r − b) + 1. 2 42 II. 2. Corollary. Let Y r ⊂ X n ⊂ PN , where X is nonsingular in a neighborhood of Y . Suppose that there is a point u ∈ PN \ X such that the projection π : X → PN −1 with center at u is a closed embedding in a neighborhood of Y . Then N ≥ n + 12 (r + 3). 3. Remark. 1 (resp. 2) is true if instead of assuming that π is a J-embedding with respect to Y (resp. an embedding in a neighborhood of Y ) we assume that π is J-unramified with respect to Y (resp.

Gv and 0, and therefore all Λi are collinear (cf. [17], [85]). On the other hand, since Gv is a cone, all Λi lie in an affine hyperplane (cf. [17]). Thus we may assume that r = 1 and Gv is the orbit of highest weight vector of an irreducible representation of a semisimple group G (varieties of such type were considered in [95] and were called HV -varieties). In particular, from this it follows that the variety X = Gv/K ∗ ⊂ PN is rational (cf. 3 below) and is defined in PN by quadratic equations (cf.

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