Une composante du bord des instantons de degré 3

In their article [1], L. Gruson and M. Skiti have constructed a birational map from the variety I₃ of mathematical instantons of degree 3 to the variety of nets of quadrics in P̌₃. They describe by this way two irreducible components of the boundary of I₃ associated to the divisor of nets which contain a two-plane degenerated quadric and the divisor of Lüroth nets. In this Note we describe an irreducible component of the boundary of I₃ as the exceptional divisor of the blowing-up of the closed set of nets of quadrics of rank 3.