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These formulas are used to derive the expressions for eigenfunctions of Laplacian in case of separation of variables, as well as to find eigenvalues and eigenvectors of multidimensional discrete Laplacian on a regular grid, which is presented as a Kronecker sum of discrete Laplacians in one-dimension.
Its eigenfunctions form a basis of the function space on which the operator is defined [5] As a consequence, in many important cases, the eigenfunctions of the Hermitian operator form an orthonormal basis. In these cases, an arbitrary function can be expressed as a linear combination of the eigenfunctions of the Hermitian operator.
The compatibility theorem tells us that a common basis of eigenfunctions of ^ and ^ can be found. Now if each pair of the eigenvalues ( a n , b n ) {\displaystyle (a_{n},b_{n})} uniquely specifies a state vector of this basis, we claim to have formed a CSCO: the set { A , B } {\displaystyle \{A,B\}} .
If the function κ is L 1 μ (X), where κ(x)=K(x,x), for all x in X, then there is an orthonormal set {e i} i of L 2 μ (X) consisting of eigenfunctions of T K such that corresponding sequence of eigenvalues {λ i} i is nonnegative. The eigenfunctions corresponding to non-zero eigenvalues are continuous on X and K has the representation
Suppose we are given a Hilbert space and a Hermitian operator over it called the Hamiltonian.Ignoring complications about continuous spectra, we consider the discrete spectrum of and a basis of eigenvectors {| } (see spectral theorem for Hermitian operators for the mathematical background): | =, where is the Kronecker delta = {, =, and the {| } satisfy the eigenvalue equation | = | .
Using the Leibniz formula for determinants, the left-hand side of equation is a polynomial function of the variable λ and the degree of this polynomial is n, the order of the matrix A. Its coefficients depend on the entries of A, except that its term of degree n is always (−1) n λ n. This polynomial is called the characteristic polynomial of A.
The covariance function K X satisfies the definition of a Mercer kernel. By Mercer's theorem, there consequently exists a set λ k, e k (t) of eigenvalues and eigenfunctions of T K X forming an orthonormal basis of L 2 ([a,b]), and K X can be expressed as
This function ω(λ) plays the role of the characteristic polynomial of D. Indeed, the uniqueness of the fundamental eigenfunctions implies that its zeros are precisely the eigenvalues of D and that each non-zero eigenspace is one-dimensional.