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Real pairing cancellation for the linearized-adjoint pair

PDF p. 19: Proof of Theorem 1.4, pairing cancellation

Statement

Assume the Axiom of Choice and the geometry, trajectory w, real cutoff φ, coefficients V1=2λ∣w∣2 and V2=λw2, and linearized and adjoint equations specified in the linearized-pair definition. Write ΔD for the Dirichlet Laplacian on L2(Ω), and (f,g)R=Re⁡∫Ωfg‾ dx.

Let z,v∈C1((0,T);L2(Ω))∩C((0,T);D(ΔD)), where the latter space has the graph norm ∥u∥D(ΔD)=(∥u∥22+∥ΔDu∥22)1/2. Suppose that h(t)∈L2(Ω) and that, for every t∈(0,T), the two equations hold in L2(Ω) with control h(t). Then

ddt(z(t),v(t))R=(−iφh(t),v(t))R(0<t<T).

In particular the Laplacian and both potential contributions cancel. The displayed minus sign on the conjugate-potential term in the adjoint equation supplies the sign needed for this cancellation.

Facts & Assumptions

[F1]

The systems in Linearized equation and its real-duality adjoint have izt+ΔDz+V1z+V2z‾=φh and ivt+ΔDv+V1v−V2v‾=0. The same definition supplies real V1, essentially bounded V1,V2 at each time, and the stated real pairing.

[F2]

Under the stated Choice assumption The Axiom of Choice, Dirichlet Laplacian and positive shift supplies the self-adjoint Dirichlet realization on D(ΔD)=H2(Ω)∩H01(Ω); Dirichlet spectral Sobolev scale and duality fixes the first-variable-linear L2 pairing. Hence ⟨ΔDf,g⟩=⟨f,ΔDg⟩ for f,g∈D(ΔD).

[F3]

The assumed full Axiom of Choice implies Countable Choice by The Axiom of Choice implies countable choice, so the Countable Choice hypothesis of Realification of Hilbert pairings and conjugate-linear coefficients is satisfied. For real essentially bounded a and complex essentially bounded b, Realification of Hilbert pairings and conjugate-linear coefficients gives the real adjoint of L:f↦i(af+bf‾) as LR∗g=−iag+ibg‾, so (Lf,g)R+(f,−LR∗g)R=0.

[F4]

The inner-product continuity estimate of The inner product is jointly continuous implies continuity of the L2 pairing under norm convergence in each argument.

Proof

[F1] The systems in Linearized equation and its real-duality adjoint have izt+ΔDz+V1z+V2z‾=φh and ivt+ΔDv+V1v−V2v‾=0. The same definition supplies real V1, essentially bounded V1,V2 at each time, and the stated real pairing.

[F2] Under the stated Choice assumption The Axiom of Choice, Dirichlet Laplacian and positive shift supplies the self-adjoint Dirichlet realization on D(ΔD)=H2(Ω)∩H01(Ω); Dirichlet spectral Sobolev scale and duality fixes the first-variable-linear L2 pairing. Hence ⟨ΔDf,g⟩=⟨f,ΔDg⟩ for f,g∈D(ΔD).

[F3] The assumed full Axiom of Choice implies Countable Choice by The Axiom of Choice implies countable choice, so the Countable Choice hypothesis of Realification of Hilbert pairings and conjugate-linear coefficients is satisfied. For real essentially bounded a and complex essentially bounded b, Realification of Hilbert pairings and conjugate-linear coefficients gives the real adjoint of L:f↦i(af+bf‾) as LR∗g=−iag+ibg‾, so (Lf,g)R+(f,−LR∗g)R=0.

[F4] The inner-product continuity estimate of The inner product is jointly continuous implies continuity of the L2 pairing under norm convergence in each argument.

Given: The stated Choice assumption, geometry, trajectory, coefficients, control, and strong solutions z,v satisfying the two equations in L2 at every time.

Goal: Prove the displayed derivative identity for each t∈(0,T).

1.1F1F3algebra

Fix t∈(0,T). The equations give zt=iΔDz+i(V1z+V2z‾)−iφh and vt=iΔDv+iV1v−iV2v‾ in L2, all evaluated at t. Set Ltf=i(V1f+V2f‾); then the potential term in vt is −Lt,R∗v. The boundedness and reality hypotheses needed for this real adjoint hold at the fixed time. The Choice implication in F3 supplies the Countable Choice hypothesis required for this application of the dualization lemma.

1.2F4given

The difference quotient of the pairing is (z(t+s)−z(t)s,v(t+s))R+(z(t),v(t+s)−v(t)s)R. The difference quotients converge in L2 to zt(t) and vt(t), and v(t+s)→v(t) in L2; pairing continuity therefore gives ddt(z,v)R=(zt,v)R+(z,vt)R.

1.3F2algebra

Self-adjointness and the first-variable-linear convention give ⟨iΔDz,v⟩=i⟨ΔDz,v⟩ and ⟨z,iΔDv⟩=−i⟨z,ΔDv⟩; their sum is zero. Thus the two Laplacian terms have zero sum also after taking real parts.

2.1F3step 1.1step 1.2step 1.3∎

The two potential terms sum to (Ltz,v)R+(z,−Lt,R∗v)R=0. Substituting the equations into the product rule and using the Laplacian cancellation leaves exactly (−iφh,v)R. Since t was arbitrary, the identity holds throughout (0,T).

Source notes

The frozen article is cited for attribution of the smooth-data identity in the proof of Theorem 1.4. The argument above uses the independent mathematical suppliers and proves the identity in the stated strong L2 class. It does not assert an extension to negative-order pivot pairings or establish existence of solutions.

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