OffDiagonalElemente

OffDiagonalElemente, commonly referred to in English as off-diagonal elements, are the entries of a square matrix that lie outside the main diagonal. For a matrix A = [a_ij], the diagonal elements satisfy i = j, while all elements with i ≠ j are off-diagonal. The diagonal part of a matrix is often denoted D, and the off-diagonal part comprises the remaining entries.

In many contexts the distinction is important. A diagonal matrix has zero off-diagonal elements, which means it scales each basis vector independently. In contrast, nonzero off-diagonal elements represent interactions or couplings between different basis components or variables. For example, in a graph adjacency matrix, off-diagonal entries indicate the presence and strength of connections between distinct nodes. In statistics, the off-diagonal elements of a covariance or correlation matrix measure pairwise relationships between variables.

Symmetry properties also involve off-diagonal elements. In a symmetric real matrix, off-diagonal entries satisfy a_ij = a_ji. In a Hermitian matrix, a_ij is the complex conjugate of a_ji. When analyzing or approximating matrices, one often separates a matrix into its diagonal and off-diagonal parts, A = D + N, where D contains the diagonal elements and N contains the off-diagonal elements. Techniques such as diagonal dominance, diagonalization, and tridiagonalization frequently address the role and magnitude of off-diagonal elements.

Beyond linear algebra, off-diagonal elements appear in diverse applications, including quantum mechanics (coherence terms in density matrices), optimization (cross-partials in Hessians), and numerical methods (Jacobi and related iterative schemes manipulate off-diagonal terms).