In mathematical analysis a pseudo-differential operator is an extension of the concept of differential operator. Pseudo-differential operators are used extensively in the theory of partial differential equations and quantum field theory.
They played an influential role in the second proof of the Atiyah–Singer index theorem via K-theory. Atiyah and Singer thanked Hörmander for assistance with understanding the theory of Pseudo-differential operators.
Linear differential operators with constant coefficients
Consider a linear differential operator with constant coefficients,
and an inverse Fourier transform, in the form:
Here, is a multi-index, are complex numbers, and
is an iterated partial derivative, where ∂j means differentiation with respect to the j-th variable. We introduce the constants to facilitate the calculation of Fourier transforms.
- Derivation of formula (1)
The Fourier transform of a smooth function u, compactly supported in Rn, is
and Fourier's inversion formula gives
By applying P(D) to this representation of u and using
one obtains formula (1).
Representation of solutions to partial differential equations
To solve the partial differential equation
we (formally) apply the Fourier transform on both sides and obtain the algebraic equation
If the symbol P(ξ) is never zero when ξ ∈ Rn, then it is possible to divide by P(ξ):
By Fourier's inversion formula, a solution is
Here it is assumed that:
- P(D) is a linear differential operator with constant coefficients,
- its symbol P(ξ) is never zero,
- both u and ƒ have a well defined Fourier transform.
The last assumption can be weakened by using the theory of distributions. The first two assumptions can be weakened as follows.
In the last formula, write out the Fourier transform of ƒ to obtain
This is similar to formula (1), except that 1/P(ξ) is not a polynomial function, but a function of a more general kind.
Definition of pseudo-differential operators
Here we view pseudo-differential operators as a generalization of differential operators. We extend formula (1) as follows. A pseudo-differential operator P(x,D) on Rn is an operator whose value on the function u(x) is the function of x:
where is the Fourier transform of u and the symbol P(x,ξ) in the integrand belongs to a certain symbol class. For instance, if P(x,ξ) is an infinitely differentiable function on Rn × Rn with the property
for all x,ξ ∈Rn, all multiindices α,β, some constants Cα, β and some real number m, then P belongs to the symbol class of Hörmander. The corresponding operator P(x,D) is called a pseudo-differential operator of order m and belongs to the class
Linear differential operators of order m with smooth bounded coefficients are pseudo-differential operators of order m. The composition PQ of two pseudo-differential operators P, Q is again a pseudo-differential operator and the symbol of PQ can be calculated by using the symbols of P and Q. The adjoint and transpose of a pseudo-differential operator is a pseudo-differential operator.
If a differential operator of order m is (uniformly) elliptic (of order m) and invertible, then its inverse is a pseudo-differential operator of order −m, and its symbol can be calculated. This means that one can solve linear elliptic differential equations more or less explicitly by using the theory of pseudo-differential operators.
Differential operators are local in the sense that one only needs the value of a function in a neighbourhood of a point to determine the effect of the operator. Pseudo-differential operators are pseudo-local, which means informally that when applied to a distribution they do not create a singularity at points where the distribution was already smooth.
Just as a differential operator can be expressed in terms of D = −id/dx in the form
for a polynomial p in D (which is called the symbol), a pseudo-differential operator has a symbol in a more general class of functions. Often one can reduce a problem in analysis of pseudo-differential operators to a sequence of algebraic problems involving their symbols, and this is the essence of microlocal analysis.
Kernel of pseudo-differential operator
Pseudo-differential operators can be represented by kernels. The singularity of the kernel on the diagonal depends on the degree of the corresponding operator. In fact, if the symbol satisfies the above differential inequalities with m ≤ 0, it can be shown that the kernel is a singular integral kernel.
- Michael E. Taylor, Pseudodifferential Operators, Princeton Univ. Press 1981. ISBN 0-691-08282-0
- M. A. Shubin, Pseudodifferential Operators and Spectral Theory, Springer-Verlag 2001. ISBN 3-540-41195-X
- Francois Treves, Introduction to Pseudo Differential and Fourier Integral Operators, (University Series in Mathematics), Plenum Publ. Co. 1981. ISBN 0-306-40404-4
- F. G. Friedlander and M. Joshi, Introduction to the Theory of Distributions, Cambridge University Press 1999. ISBN 0-521-64971-4
- Hörmander, Lars (1987). The Analysis of Linear Partial Differential Operators III: Pseudo-Differential Operators. Springer. ISBN 3-540-49937-7.