Pseudo-differential operator

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.
History.
The study of pseudo-differential operators began in the mid 1960s with the work of Kohn, Nirenberg, Hörmander, Unterberger and Bokobza.
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.
Motivation.
Linear differential operators with constant coefficients.
Consider a linear differential operator with constant coefficients,
which acts on smooth functions formula_2 with compact support in R"n".
This operator can be written as a composition of a Fourier transform, a simple "multiplication" by the
polynomial function (called the symbol)
and an inverse Fourier transform, in the form:
Here, formula_4 is a multi-index, formula_5 are complex numbers, and
is an iterated partial derivative, where ∂"j" means differentiation with respect to the "j"-th variable. We introduce the constants formula_7 to facilitate the calculation of Fourier transforms.
The Fourier transform of a smooth function "u", compactly supported in R"n", is
and Fourier's inversion formula gives
By applying "P"("D") to this representation of "u" and using
one obtains formula ().
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 ∈ R"n", then it is possible to divide by "P"(ξ):
By Fourier's inversion formula, a solution is
Here it is assumed that:
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 (), 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 R"n" is an operator whose value on the function "u(x)" is the function of "x":
where formula_16 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 R"n" × R"n" with the property
for all "x",ξ ∈R"n", all multiindices α,β, some constants "C"α, β and some real number "m", then "P" belongs to the symbol class formula_18 of Hörmander. The corresponding operator "P"("x","D") is called a pseudo-differential operator of order m and belongs to the class
formula_19
Properties.
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/d"x" 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. 