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Lemma 3.11: extension preserving the obstacle Dirichlet condition

PDF p. 16: Lemma 3.11

Statement

Assume the Axiom of Choice. Let n≥2, let Θ⊂Rn be nonempty and compact with smooth boundary, and suppose its exterior Ω=Rn∖Θ is connected. Choose R0>0 with Θ⊂BR0(0) and put Ω0=BR0(0)∖Θ and Γ2=∂Θ. For 1≤s≤2, let Xs(Ω0)={u∈Hs(Ω0):γΓ2u=0} with the inherited Hs norm, where Hs is the Bessel-potential restriction space and γΓ2 is the value trace. Let HDs(Ω)=D((I−ΔD)s/2) with its spectral norm, where ΔD is the Dirichlet Laplacian. Then there is a bounded linear operator E:Xs(Ω0)→HDs(Ω) such that (Eu)∣Ω0=u for every u∈Xs(Ω0).

Facts & Assumptions

Given: The hypotheses and conventions in the Statement.

[F1]

The geometry and spaces are those of Exterior and truncated obstacle domains, States compatible with the uncontrolled obstacle boundary and Dirichlet spectral Sobolev scale and duality. The Choice assumption is The Axiom of Choice. Bounded linear operators between the resulting complex normed spaces are understood as in A bounded linear operator between normed spaces.

[F2]

The fixed annular construction Annular extension and cutoff gluing construction, with the partial-extension and gluing result Partial extension across an artificial sphere and annular cutoff gluing, gives a bounded linear map G:Hs(Ω0)→Hs(Ω) which restricts to the identity on Ω0 and maps Xs(Ω0) into Hs(Ω)∩H01(Ω). Here H01(Ω) is the H1(Ω) closure of Cc∞(Ω).

[F3]

Under the stated geometry and Choice assumption, Low-order identifications of the Dirichlet scale identifies HDs(Ω) with Hs(Ω)∩H01(Ω) for 1≤s≤2, with the spectral and inherited Hs norms equivalent.

Proof

Given: The hypotheses and conventions in the Statement.

[F1] The geometry and spaces are those of Exterior and truncated obstacle domains, States compatible with the uncontrolled obstacle boundary and Dirichlet spectral Sobolev scale and duality. The Choice assumption is The Axiom of Choice. Bounded linear operators between the resulting complex normed spaces are understood as in A bounded linear operator between normed spaces.

[F2] The fixed annular construction Annular extension and cutoff gluing construction, with the partial-extension and gluing result Partial extension across an artificial sphere and annular cutoff gluing, gives a bounded linear map G:Hs(Ω0)→Hs(Ω) which restricts to the identity on Ω0 and maps Xs(Ω0) into Hs(Ω)∩H01(Ω). Here H01(Ω) is the H1(Ω) closure of Cc∞(Ω).

[F3] Under the stated geometry and Choice assumption, Low-order identifications of the Dirichlet scale identifies HDs(Ω) with Hs(Ω)∩H01(Ω) for 1≤s≤2, with the spectral and inherited Hs norms equivalent.

Given: The hypotheses and data in the Statement.

Goal: Construct a bounded linear right inverse to restriction with values in HDs(Ω).

1.1F2given

Fix the cutoffs and annular extension in the construction of G. Restrict its domain to Xs(Ω0) and call the resulting map E. The gluing result gives Eu∈Hs(Ω)∩H01(Ω) and (Eu)∣Ω0=u. It also gives a constant Cs such that ∥Eu∥Hs(Ω)≤Cs∥u∥Hs(Ω0). The fixed construction is linear.

2.1F1F3step 1.1∎

The scale identification gives a constant Cs′ such that ∥Eu∥HDs(Ω)≤Cs′∥Eu∥Hs(Ω) on Hs(Ω)∩H01(Ω). Combining this with the previous bound and the inherited norm of Xs(Ω0) proves that E is bounded into HDs(Ω). Its restriction identity and linearity were established above.

Source notes

The original article is cited for attribution of Lemma 3.11 and its annular construction. The proof here applies the separately supplied gluing and spectral-identification results; the article's assertion is not used as a premise.

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