Hyperfunction

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In mathematics, hyperfunctions are generalizations of functions, as a 'jump' from one holomorphic function to another at a boundary, and can be thought of informally as distributions of infinite order. Hyperfunctions were introduced by Mikio Sato in 1958, building upon earlier work by Grothendieck and others.

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[edit] Formulation

We want a hyperfunction on the real line to be the 'difference' between one holomorphic function on the upper half-plane and another on the lower half-plane. The easiest way to achieve this is to say that a hyperfunction is specified by a pair (f, g), where f is a holomorphic function on the upper half-plane and g is a holomorphic function on the lower half-plane.

Informally, the hyperfunction is what the difference f - g would be at the real line itself. This difference is not affected by adding the same holomorphic function to both f and g, so if h is a holomorphic function on the whole complex plane, the hyperfunctions (f, g) and (f+h, g+h) are defined to be equivalent.

[edit] Definition in one dimension

The motivation can be concretely implemented using ideas from sheaf cohomology. Let \mathcal{O} be the sheaf of holomorphic functions on C. Define the hyperfunctions on the real line by

\mathcal{B}(\mathbf{R}) = H^1_{\mathbf{R}}(\mathbf{C}, \mathcal{O}),

the first local cohomology group.

Concretely, let C+ and C− be the upper half-plane and lower half-plane respectively. Then

\mathbf{C}^+ \cup \mathbf{C}^- = \mathbf{C} \setminus \mathbf{R}.\,

so

H^1_{\mathbf{R}}(\mathbf{C}, \mathcal{O}) = \left [ H^0(\mathbf{C}^+, \mathcal{O}) \oplus H^0(\mathbf{C}^-, \mathcal{O}) \right ] /H^0(\mathbf{C}, \mathcal{O}).

Since the zeroth cohomology group of any sheaf is simply the global sections of that sheaf, we see that a hyperfunction is a pair of holomorphic functions one each on the upper and lower complex halfplane modulo entire holomorphic functions.

[edit] Examples

  • If f is any holomorphic function on the whole complex plane, then the restriction of f to the real axis is a hyperfunction, represented by either (f, 0) or (0, -f).
  • If g is a continuous function (or more generally a distribution) on the real line with support contained in a bounded interval I, then g corresponds to the hyperfunction (f, −f), where f is a holomorphic function on the complement of I defined by
f(z)={1\over 2\pi i}\int_{x\in I} g(x){dx\over z-x}.
This function f jumps in value by g(x) when crossing the real axis at the point x. The formula for f follows from the previous example by writing g as the convolution of itself with the Dirac delta function.
  • If f is any function that is holomorphic everywhere except for an essential singularity at 0 (for example, e1/z), then (f, −f) is a hyperfunction with support 0 that is not a distribution. If f has a pole of finite order at 0 then (f, −f) is a distribution, so when f has an essential singularity then (f,−f) looks like a "distribution of infinite order" at 0. (Note that distributions always have finite order at any point.)

[edit] References

  • Hörmander, Lars (2003), The analysis of linear partial differential operators, Volume I: Distribution theory and Fourier analysis, Berlin: Springer-Verlag, ISBN 3-540-00662-1 .