The complex shade of a real space and its applications

Abstract

A natural oriented (2k+2)-chain in CP^{2k+1} with boundary twice RP^{2k+1}, its complex shade, is constructed. Via intersection numbers with the shade, a new invariant, the shade number of k-dimensional subvarieties with normal vector fields along their real part, is introduced. For an even-dimensional real variety, the shade number and the Euler number of the complement of the normal vector field in the real normal bundle of its real part agree. For an odd- dimensional orientable real variety, a linear combination of the shade number and the wrapping number (self-linking number) of its real part is independent of the normal vector field and equals the encomplexed writhe as defined by Viro. Shade numbers of varieties without real points and encomplexed writhes of odd- dimensional real varieties are, in a sense, Vassiliev invariants of degree 1. Complex shades of odd-dimensional spheres are constructed. Shade numbers of real subvarieties in spheres have properties analogous to those of their projective counterparts.