In mathematics, a biorthogonal system is a pair of indexed families of vectors such that where and form a pair of topological vector spaces that are in duality, is a bilinear mapping and is the Kronecker delta. An example is the pair of sets of respectively left and right eigenvectors of a matrix, indexed by eigenvalue, if the eigenvalues are distinct. A biorthogonal system in which and is an orthonormal system. Projection Related to a biorthogonal system is the projection where its image is the linear span of and the kernel is Construction Given a possibly non-orthogonal set of vectors and the projection related is where is the matrix with entries and then is a biorthogonal system. See also References Jean Dieudonné, On biorthogonal systems Michigan Math. J. 2 (1953), no. 1, 7–20 Topological vector spaces