Physlib

PhyslibAlpha.ProbabilisticTheory.HilbertSpace.TraceClass.Banach

The trace-class Banach space

i. Overview

The trace-class operators form a Banach space `𝒮₁(H)` under the trace norm. The trace norm dominates the operator norm, so an absolutely convergent series of trace-class operators converges in operator norm. Its limit is trace class because the trace norm is lower semicontinuous along such limits, which gives completeness.

ii. Key results

- `TraceClass` : the trace-class operators with the trace norm. - `opNorm_le_traceNorm` : the operator norm is at most the trace norm. - `isTraceClass_of_tendsto_of_traceNorm_bounded` : an operator-norm limit of trace-class operators with bounded trace norms is trace class. - `TraceClass.instCompleteSpace` : `𝒮₁(H)` is complete.

iii. Table of contents

  • A. Arithmetic closure of `IsTraceClass`
  • B. The trace-class submodule and Banach space

A. Arithmetic closure of `IsTraceClass`

B. The trace-class submodule and Banach space

21 declarations

theorem

The zero operator is trace-class

The zero operator 0:H→H0 : H \to H on a Hilbert space HH is trace-class.

theorem

Scalar multiples of trace-class operators are trace-class

Let HH be a complex Hilbert space and T:H→HT: H \to H be a continuous linear operator. For any complex scalar c∈Cc \in \mathbb{C}, if TT is a trace-class operator, then the scalar multiple c⋅Tc \cdot T is also a trace-class operator.

theorem

If TT is trace-class, then −T-T is trace-class

Let HH be a complex Hilbert space and T:H→HT: H \to H be a bounded linear operator. If TT is a trace-class operator, then its negation −T-T is also a trace-class operator.

theorem

∥0∥1=0\|0\|_1 = 0 for the zero operator

Let HH be a complex Hilbert space. The trace norm of the zero operator 0:H→H0 : H \to H is equal to 00, i.e., ∥0∥1=0\|0\|_1 = 0.

theorem

Absolute Homogeneity of the Trace Norm: ∥c⋅T∥1=∣c∣∥T∥1\|c \cdot T\|_1 = |c| \|T\|_1

Let HH be a complex Hilbert space and T:H→HT: H \to H be a continuous linear operator. For any complex scalar c∈Cc \in \mathbb{C}, if TT is a trace-class operator, then the trace norm of the scalar multiple c⋅Tc \cdot T is equal to the product of the absolute value of cc and the trace norm of TT: ∥c⋅T∥1=∣c∣∥T∥1\|c \cdot T\|_1 = |c| \|T\|_1 where ∥⋅∥1\|\cdot\|_1 denotes the trace norm.

theorem

∥−T∥1=∥T∥1\| -T \|_1 = \| T \|_1 for trace-class operators

Let HH be a complex Hilbert space and let T:H→HT: H \to H be a continuous linear operator. If TT is a trace-class operator, then its negation −T-T is also a trace-class operator, and the trace norm of −T-T is equal to the trace norm of TT, i.e., ∥−T∥1=∥T∥1\| -T \|_1 = \| T \|_1.

theorem

Operator Norm is Bounded by Trace Norm: ∥T∥≤∥T∥1\|T\| \le \|T\|_1

For any trace-class operator TT on a complex Hilbert space HH, the operator norm ∥T∥\|T\| is less than or equal to the trace norm ∥T∥1\|T\|_1.

definition

C\mathbb{C}-submodule of trace-class operators on HH

Let HH be a complex Hilbert space and let B(H)\mathcal{B}(H) (represented as `H →L[ℂ] H`) be the space of bounded linear operators on HH. The `traceClassSubmodule` is the C\mathbb{C}-submodule of B(H)\mathcal{B}(H) consisting of all trace-class operators. It is defined by showing that the zero operator is trace-class, and that the property of being trace-class is preserved under operator addition and scalar multiplication by complex numbers C\mathbb{C}.

definition

Trace-class Banach space S1(H)\mathcal{S}_1(H)

Let HH be a complex Hilbert space. The trace-class Banach space S1(H)\mathcal{S}_1(H) is the space consisting of all trace-class bounded linear operators on HH, equipped with the trace norm ∥⋅∥1\| \cdot \|_1.

instance

S1(H)\mathcal{S}_1(H) is an additive abelian group

Let HH be a complex Hilbert space. The space of trace-class operators S1(H)\mathcal{S}_1(H) forms an additive abelian group under the standard addition of operators. This structure provides the definition for the sum of two trace-class operators, the zero operator, and the additive inverse (negation) of a trace-class operator within the space S1(H)\mathcal{S}_1(H).

instance

S1(H)\mathcal{S}_1(H) is a vector space over C\mathbb{C}

Let HH be a complex Hilbert space. The space of trace-class operators S1(H)\mathcal{S}_1(H) forms a vector space (module) over the field of complex numbers C\mathbb{C}. This structure defines the scalar multiplication of a trace-class operator by a complex number, such that for any c∈Cc \in \mathbb{C} and T∈S1(H)T \in \mathcal{S}_1(H), the operator cTcT is also an element of S1(H)\mathcal{S}_1(H).

theorem

T∈traceClassSubmodule  ⟺  IsTraceClass TT \in \text{traceClassSubmodule} \iff \text{IsTraceClass } T

Let HH be a complex Hilbert space and T∈B(H)T \in \mathcal{B}(H) be a bounded linear operator. TT belongs to the trace-class submodule of B(H)\mathcal{B}(H) if and only if TT is a trace-class operator.

theorem

Elements of S1(H)\mathcal{S}_1(H) are trace-class operators

Let HH be a complex Hilbert space and let S1(H)\mathcal{S}_1(H) (denoted in the formal text as `TraceClass H`) be the Banach space of trace-class operators on HH. For any element T∈S1(H)T \in \mathcal{S}_1(H), the underlying bounded linear operator is a trace-class operator.

definition

Construction of an element in S1(H)\mathcal{S}_1(H) from a trace-class operator TT

Given a bounded linear operator T∈B(H)T \in \mathcal{B}(H) on a complex Hilbert space HH and a proof hTh_T that TT is trace-class (i.e., TT satisfies `IsTraceClass T`), this function returns TT as an element of the trace-class Banach space S1(H)\mathcal{S}_1(H).

theorem

The underlying operator of a constructed trace-class element is the original operator

Let HH be a complex Hilbert space and B(H)\mathcal{B}(H) be the space of bounded linear operators on HH. For any operator T∈B(H)T \in \mathcal{B}(H) that is trace-class, the underlying bounded linear operator of the element in the trace-class space S1(H)\mathcal{S}_1(H) constructed from TT (via `ofOperator`) is equal to TT.

definition

Trace norm ∥⋅∥1\| \cdot \|_1 is an `AddGroupNorm` on S1(H)\mathcal{S}_1(H)

Let HH be a complex Hilbert space and let S1(H)\mathcal{S}_1(H) be the space of trace-class operators on HH. The trace norm ∥⋅∥1\|\cdot\|_1 satisfies the axioms of an additive group norm (`AddGroupNorm`) on S1(H)\mathcal{S}_1(H). Specifically, for all operators T,T′∈S1(H)T, T' \in \mathcal{S}_1(H), the following properties hold: - **Identity**: ∥0∥1=0\|0\|_1 = 0. - **Negation**: ∥−T∥1=∥T∥1\|-T\|_1 = \|T\|_1. - **Triangle Inequality**: ∥T+T′∥1≤∥T∥1+∥T′∥1\|T + T'\|_1 \leq \|T\|_1 + \|T'\|_1. - **Definiteness**: ∥T∥1=0\|T\|_1 = 0 implies T=0T = 0. This is established by the fact that the operator norm is dominated by the trace norm (∥T∥≤∥T∥1\|T\| \leq \|T\|_1), so if the trace norm vanishes, the operator itself must be zero.

instance

S1(H)\mathcal{S}_1(H) is a normed additive abelian group

Let HH be a complex Hilbert space. The space of trace-class operators S1(H)\mathcal{S}_1(H) is a normed additive abelian group, where the norm of an operator T∈S1(H)T \in \mathcal{S}_1(H) is defined as the trace norm ∥T∥1\|T\|_1.

theorem

The norm on S1(H)\mathcal{S}_1(H) is the trace norm ∥⋅∥1\| \cdot \|_1

Let HH be a complex Hilbert space and let S1(H)\mathcal{S}_1(H) denote the Banach space of trace-class operators on HH. For any operator T∈S1(H)T \in \mathcal{S}_1(H), the norm ∥T∥\|T\| in the space S1(H)\mathcal{S}_1(H) is equal to the trace norm ∥T∥1\|T\|_1.

instance

S1(H)\mathcal{S}_1(H) is a normed space over C\mathbb{C}

Let HH be a complex Hilbert space. The space of trace-class operators S1(H)\mathcal{S}_1(H) is a normed space over the field of complex numbers C\mathbb{C} with respect to the trace norm ∥⋅∥1\|\cdot\|_1. This structure implies that scalar multiplication is compatible with the norm, such that for any c∈Cc \in \mathbb{C} and T∈S1(H)T \in \mathcal{S}_1(H), the identity ∥cT∥1=∣c∣∥T∥1\|c T\|_1 = |c| \|T\|_1 holds.

theorem

Lower Semicontinuity of the Trace Norm with respect to Operator-Norm Convergence

Let HH be a Hilbert space. Let (Tn)n∈N(T_n)_{n \in \mathbb{N}} be a sequence of continuous linear operators on HH, and let TT be a continuous linear operator on HH. Suppose that for every nn, TnT_n is a trace-class operator whose trace norm is bounded by a constant CC (i.e., ∥Tn∥1≤C\|T_n\|_1 \le C for all nn). If TnT_n converges to TT in the operator norm as n→∞n \to \infty, then TT is also a trace-class operator and its trace norm satisfies ∥T∥1≤C\|T\|_1 \le C.

instance

The trace-class space S1(H)\mathcal{S}_1(H) is a complete space

Let HH be a complex Hilbert space. The space of trace-class operators S1(H)\mathcal{S}_1(H) on HH, equipped with the trace norm ∥⋅∥1\|\cdot\|_1, is a complete metric space. Consequently, since S1(H)\mathcal{S}_1(H) is a normed space, it is a Banach space.