Some calculations on type II_1 unprojection

Abstract

The type II_1 unprojection is, by definition, the generic complete intersection type II unprojection, in the sense of [Papadakis, Type II unprojection, J. Algebraic Geometry, 15 (2006) 399--414] Section 3.1, for the parameter value k = 1, and depends on a parameter n greater or equal than 2. Our main results are the explicit calculation of the linear relations of the type II_1 unprojection for any value of n greater or equal than 2 (Theorem 3.16) and the explicit calculation of the quadratic equation for the case n = 3 (Theorem 4.1). In addition, Section 5 contains applications to algebraic geometry while Section 6 contains the Macaulay 2 code for the type II_1 unprojection for the parameter value n = 3.