Rank and eigenvalues of a supersymmetric tensor, the multivariate homogeneous polynomial and the algebraic hypersurface it defines

• Authors: Liqun Qi
• Venue: Journal of Symbolic Computation
• Year: 2006

A real n-dimensional homogeneous polynomial f(x) of degree m and a real constant c define an algebraic hypersurface S whose points satisfy f(x)=c. The polynomial f can be represented by Ax^m where A is a real mth order n-dimensional supersymmetric tensor. In this paper, we define rank, base index and eigenvalues for the polynomial f, the hypersurface S and the tensor A. The rank is a nonnegative integer r less than or equal to n. When r is less than n, A is singular, f can be converted into a homogeneous polynomial with r variables by an orthogonal transformation, and S is a cylinder hypersurface whose base is r-dimensional. The eigenvalues of f, A and S always exist. The eigenvectors associated with the zero eigenvalue are either recession vectors or degeneracy vectors of positive degree, or their sums. When c@?=0, the eigenvalues with the same sign as c and their eigenvectors correspond to the characterization points of S, while a degeneracy vector generates an asymptotic ray for the base of S or its conjugate hypersurface. The base index is a nonnegative integer d less than m. If d=k, then there are nonzero degeneracy vectors of degree k-1, but no nonzero degeneracy vectors of degree k. A linear combination of a degeneracy vector of degree k and a degeneracy vector of degree j is a degeneracy vector of degree k+j-m if k+j=m. Based upon these properties, we classify such algebraic hypersurfaces in the nonsingular case into ten classes.