nLab one-parameter semigroup

Contents

Context

Group Theory

Operator algebra

algebraic quantum field theory (perturbative, on curved spacetimes, homotopical)

Introduction

Concepts

field theory:

Lagrangian field theory

quantization

quantum mechanical system, quantum probability

free field quantization

gauge theories

interacting field quantization

renormalization

Theorems

States and observables

Operator algebra

Local QFT

Perturbative QFT

Contents

1. Idea

A one-parameter group (of unitary operators in a Hilbert space) is a homomorphism of groups

RU(H),\mathbf{R} \to U(H),

where HH is a Hilbert spaces, U ( H ) U(H) denotes its group of unitary operators and R\mathbf{R} the additive group of real numbers.

More generally, one can define one-parameter semigroups of operators in a Banach space XX as homomomorphisms of monoids

R 0B(X),\mathbf{R}_{\ge0} \to B(X),

where B(X)B(X) denotes the semigroup of bounded operators XXX\to X.

Typically, we also require a continuity condition such as continuity in the strong topology.

2. Stone theorem

Strongly continuous one-parameter unitary groups (U t) t0(U_t)_{t\ge0} of operators in a Hilbert space HH are in bijection with self-adjoint unbounded operators AA on HH:

This bijection sends

A(texp(itA)). A \mapsto \big( t\mapsto \exp(itA) \big) \,.

The operator AA is bounded if and only if UU is norm-continuous.

3. Hille–Yosida theorem

Strongly continuous one-parameter semigroups TT of bounded operators on a Banach space XX (alias C 0C_0-semigroups) satisfying T(t)Mexp(ωt)\|T(t)\|\le M\exp(\omega t) are in bijection with closed operators A:XXA\colon X\to X with dense domain such that any λ>ω\lambda\gt \omega belongs to the resolvent set of AA and for any λ>ω\lambda\gt\omega we have

(λIA) nM(λω) n.\|(\lambda I-A)^{-n}\|\le M (\lambda-\omega)^{-n}.

5. References

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See also

Last revised on June 21, 2022 at 07:35:23. See the history of this page for a list of all contributions to it.