Elliptic operator

In the theory of partial differential equations, elliptic operators are differential operators that generalize the Laplace operator. They are defined by the condition that the coefficients of the highest-order derivatives be positive, which implies the key property that the principal symbol is invertible, or equivalently that there are no real characteristic directions. Elliptic operators are typical of potential theory, and they appear frequently in electrostatics and continuum mechanics. Elliptic regularity implies that their solutions tend to be smooth functions (if the coefficients in the operator are smooth). Steady-state solutions to hyperbolic and parabolic equations generally solve elliptic equations. Let be a linear differential operator of order m on a domain in Rn given by where denotes a multi-index, and denotes the partial derivative of order in . Then is called elliptic if for every x in and every non-zero in Rn, where . In many applications, this condition is not strong enough, and instead a uniform ellipticity condition may be imposed for operators of order m = 2k: where C is a positive constant. Note that ellipticity only depends on the highest-order terms. A nonlinear operator is elliptic if its linearization is; i.e. the first-order Taylor expansion with respect to u and its derivatives about any point is an elliptic operator. Example 1 The negative of the Laplacian in Rd given by is a uniformly elliptic operator. The Laplace operator occurs frequently in electrostatics. If ρ is the charge density within some region Ω, the potential Φ must satisfy the equation Example 2 Given a matrix-valued function A(x) which is symmetric and positive definite for every x, having components aij, the operator is elliptic. This is the most general form of a second-order divergence form linear elliptic differential operator. The Laplace operator is obtained by taking A = I. These operators also occur in electrostatics in polarized media.