The normal Euler class and singularities of projections for polyhedral surfaces in 4-space
The authors' summary: ``This paper defines the normal Euler number and the normal Euler class for polyhedral surfaces in 4-space by means of singularities of projections into hyperplanes. There exist polyhedral analogues of nearly all of Whitney's theorems on normal Euler classes of surfaces smoothly immersed in 4-space. However, although the normal Euler number of a smooth embedding of the real projective plane in 4-space must be plus or minus 2, the normal Euler number of a (locally knotted) polyhedral embedding of the real projective plane can be any integer congruent to 2 modulo 4''.
Whitney's theorems on normal Euler classes of surfaces smoothly immersed in 4-space
scientific article; zbMATH DE number 1113444