Analytic semigroup

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In mathematics, an analytic semigroup is particular kind of strongly continuous semigroup. Analytic semigroups are used in the solution of partial differential equations; compared to strongly continuous semigroups, analytic semigroups provide better regularity of solutions to initial value problems, better results concerning perturbations of the infinitesimal generator, and a relationship between the type of the semigroup and the spectrum of the infinitesimal generator.

Definition[edit]

Let Γ(t) = exp(At) be a strongly continuous one-parameter semigroup on a Banach space (X, ||·||) with infinitesimal generator A. Γ is said to be an analytic semigroup if

and the usual semigroup conditions hold for st ∈ Δθ: exp(A0) = id, exp(A(t + s)) = exp(At)exp(As), and, for each x ∈ X, exp(At)x is continuous in t;

Characterization[edit]

The infinitesimal generators of analytic semigroups have the following characterization:

A closed, densely defined linear operator A on a Banach space X is the generator of an analytic semigroup if and only if there exists an ω ∈ R such that the half-plane Re(λ) > ω is contained in the resolvent set of A and, moreover, there is a constant C such that

for Re(λ) > ω and where is the resolvent of the operator A. Such operators are called sectorial. If this is the case, then the resolvent set actually contains a sector of the form

for some δ > 0, and an analogous resolvent estimate holds in this sector. Moreover, the semigroup is represented by

where γ is any curve from e∞ to e+∞ such that γ lies entirely in the sector

with π ⁄ 2 < θ < π ⁄ 2 + δ.

References[edit]

  • Renardy, Michael; Rogers, Robert C. (2004). An introduction to partial differential equations. Texts in Applied Mathematics 13 (Second ed.). New York: Springer-Verlag. pp. xiv+434. ISBN 0-387-00444-0. MR 2028503.