# Microlocal analysis

Microlocal analysis considers (generalized, hyper-) functions, operators, etc. in the "microlocal" range. Here, "microlocal" means seeing the matter more locally than usual by introducing the (cotangential) direction at every point. In Fourier analysis it corresponds to viewing things locally in both and . In view of the uncertainty principle, this is possible only by considering the objects modulo regular parts. This idea was first used in the study of pseudo-differential operators by P.D. Lax, S. Mizohata, L. Hörmander, etc. V.P. Maslov has enriched the theory by the introduction of a canonical structure. M. Sato has constructed the sheaf of micro-functions on the cotangent sphere bundle of the base space as the basic object of microlocal analysis.

## Micro-analyticity of hyperfunctions.

A hyperfunction is said to be micro-analytic at if on a neighbourhood of it admits analytic continuation into the half-space , in the sense that it admits a boundary-value representation such that for every . This is equivalent to saying that near , , where is the germ of a real-analytic function and is a Fourier hyperfunction (cf. Hyperfunction) exponentially decreasing in a conic neighbourhood of . The set of points at which is not micro-analytic is called the singular spectrum of , and is denoted by . By definition,

where is the dual cone of . Conversely, a hyperfunction satisfying this estimate can be written in the form .

## Operations and the singular spectrum.

The following inclusions hold:

Here, the operations are legitimate if the vector does not appear in the -component of the right-hand side. In particular, under coordinate transformation the singular spectrum behaves like a subset of . Restriction is possible if , in which case is said to contain as real-analytic parameter at , and then

where denotes projection on the components. The dual assertion is:

The combination of these assertions gives a convolution, dual to the product:

Let be a linear differential operator with real-analytic coefficients and let be its characteristic manifold. Then

(Sato's fundamental theorem). Hence Cauchy data for solutions can be specified on a non-characteristic manifold. Holmgren's uniqueness theorem holds with these data. More generally, for any hyperfunction , implies (the Kashiwara–Kawai Holmgren-type theorem); further, the fibre of at has the form , where denotes projection to the equator (the watermelon theorem).

## Decomposition of singular spectra.

One has

where the twisted phase is a real-analytic function of which is of positive type in (that is, implies ), is homogeneous of degree 1 in , and , ; and the vector is such that . This is a generalization of the classical Radon decomposition, in which

The component, regarded as a hyperfunction of , has a singular spectrum with only one direction . Via convolution it gives a similar decomposition of general hyperfunctions. If also satisfies for , then the singular spectrum of the component as a hyperfunction of is precisely one point ; this is useful in applications. Typical examples are:

(Kashiwara example);

(Bony example). For such a decomposition is micro-analytic at if and only if is real analytic in at .

The Fourier–Bros–Iagolnitzer transform of a hyperfunction (the FBI-transform of ) is

where is a real-analytic function satisfying: 1) for one has and ; and 2) for some . A typical example of such a is

A hyperfunction is micro-analytic at if and only if for some (equivalently, any) modification of with compact support its FBI-transform is exponentially decreasing with respect to uniformly in in a neighbourhood of . Integration of the inversion formula over the radial variable gives the formula

This supplies a partition of unity of the sheaf . All these arguments are compatible with the corresponding theory of (analytic) wave front sets for distributions.

The sheaf of micro-functions on is the sheaf associated with the pre-sheaf

The sequence of sheaves on :

is exact. Here denotes projection. By definition, for a hyperfunction . The sheaf is flabby, which implies the possibility of arbitrary modification of hyperfunctions preserving the singular spectrum. Analytic pseudo-differential operators (cf. Pseudo-differential operator) and micro-differential operators naturally act on as sheaf homomorphisms. They act isomorphically at a non-characteristic point. Canonical transforms induce ring isomorphisms of the sheaf of pseudo-differential operators. Using these, a simple characteristic system of pseudo-differential equations can be locally reduced to the direct sum of copies of de Rham, Cauchy–Riemann and Lewy–Mizohata equations (the fundamental structure theorem).

The sheaf is constructed from the sheaf by Sato micro-localization: Let be a real-analytic manifold and a complex neighbourhood of it. Let be the real blowing-up, , the canonical inclusions, and let be the subset of the fibre product of and over defined by ; let be the canonical projections to the factors. Let be the orientation sheaf of . Then

(the fundamental vanishing theorem). This argument can be generalized to the second micro-localization of micro-functions with respect to a holomorphic parameter or to micro-localization of any sheaf.