Smooth Complete Intersections with Positive-Definite Intersection Form

Abstract

We classify the smooth complete intersections with positive-definite intersection form on their middle cohomology. There are two families. The first family are quadric hypersurfaces in P(4k+1) with k a positive integer. The middle cohomology is always of rank two and the intersection lattice corresponds to the identity matrix. The second family are complete intersections of two quadrics in P(4k+2) (k a positive integer). Here the intersection lattices are the Gamma(4(k+1)) lattices; in particular, the intersection lattice of a smooth complete intersection of two quadrics in P(6) is the famous E8 lattice.