Threefolds of P5 with one apparent quadruple point
Abstract
In this article we classify all the smooth threefolds of P5 with an apparent quadruple point provided that the family of its 4-secant lines is an irreducible (first order) congruence. This is sufficient to conclude the classification of all the smooth codimension two varieties of Pn with one apparent (n-1)-point and with irreducible family of (n-1)-secant lines.
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