Weakly defective varieties

Transactions of the American Mathematical Society - Tập 354 Số 1 - Trang 151-178
Luca Chiantini1,2, Ciro Ciliberto1,2
1Department of Mathematics, University of Rome II, Viale della Ricerca Scientifica, 16132 Rome, Italy
2Department of Mathematics, University of Siena, Via del Capitano 15, 53100 Siena, Italy

Tóm tắt

A projective variety X X is ‘ k k -weakly defective’ when its intersection with a general ( k + 1 ) (k+1) -tangent hyperplane has no isolated singularities at the k + 1 k+1 points of tangency. If X X is k k -defective, i.e. if the k k -secant variety of X X has dimension smaller than expected, then X X is also k k -weakly defective. The converse does not hold in general. A classification of weakly defective varieties seems to be a basic step in the study of defective varieties of higher dimension. We start this classification here, describing all weakly defective irreducible surfaces. Our method also provides a new proof of the classical Terracini’s classification of k k -defective surfaces.

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