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Cubic and quintic energy-space nonlinear time estimates

PDF p. 15: time-integrated estimate in Proposition 3.9; PDF p. 16: opening difference estimate

Statement

Assume the Axiom of Choice. Let n≥2, let Θ⊂Rn be nonempty and compact with smooth boundary, and suppose that Ω=Rn∖Θ is connected. Let T>0, m∈{3,5} and λ∈R∖{0}, and set g(z)=λ∣z∣m−1z.

Let H01(Ω) be the closure of Cc∞(Ω;C) in the norm ∥w∥H1=(∥w∥22+∑j∥Djw∥22)1/2. Define XT=C([0,T];H01(Ω))∩Lt4Lx∞ using compatible jointly measurable representatives and the norm ∥w∥XT=sup⁡t∈[0,T]∥w(t)∥H1+∥w∥Lt4Lx∞. The mixed norm is the L4(0,T) norm of the spatial essential supremum; it does not require strong measurability into L∞(Ω).

For u,v∈XT, both g(u) and g(v) belong to the Bochner space L1(0,T;H01(Ω)), and

∥g(v)∥Lt1H01≤m∣λ∣T(5−m)/4∥v∥XTm.

There is a constant C depending only on m and ∣λ∣ such that

∥g(v)−g(u)∥Lt1H01≤CT(5−m)/4(∥v∥XTm−1+∥u∥XTm−1)∥v−u∥XT.

For m=5 the factor involving T is 1.

Facts & Assumptions

[F1]

The energy-space definition supplies the compatible representative convention and the displayed XT norm; the integer Sobolev and time-space definitions supply the weak-derivative norm, closure and Bochner and mixed-norm conventions. Energy control space with a time-integrable supremum norm Complex integer-order Sobolev notation Continuous and mixed time-space function spaces

[F2]

For bounded w∈H01(Ω), the power chain rule gives g(w)∈H01(Ω) and ∥g(w)∥H1≤m∣λ∣∥w∥∞m−1∥w∥H1. The pointwise-differences proposition gives the corresponding energy difference bound with one term containing ∥v−u∥H1 and one containing ∥v−u∥∞. Power nonlinearity preserves the zero-boundary energy space Pointwise and energy difference estimates for odd powers

[F3]

The time-integrability lemma applies to the stated compatible mixed-norm convention and establishes strong H01 measurability and L1 membership for the nonlinear terms, without assuming Bochner measurability into L∞. Cubic and quintic time bounds from L4 Linfinity control

[F4]

Hölder's integral inequality applies to nonnegative measurable time functions, including its 1,∞ endpoint. Holder's inequality for integrals, including the endpoint cases

[F5]

The assumed Axiom of Choice supplies the choice hypotheses carried by these inputs. The Axiom of Choice

Proof

[F1] The energy-space definition supplies the compatible representative convention and the displayed XT norm; the integer Sobolev and time-space definitions supply the weak-derivative norm, closure and Bochner and mixed-norm conventions. Energy control space with a time-integrable supremum norm Complex integer-order Sobolev notation Continuous and mixed time-space function spaces

[F2] For bounded w∈H01(Ω), the power chain rule gives g(w)∈H01(Ω) and ∥g(w)∥H1≤m∣λ∣∥w∥∞m−1∥w∥H1. The pointwise-differences proposition gives the corresponding energy difference bound with one term containing ∥v−u∥H1 and one containing ∥v−u∥∞. Power nonlinearity preserves the zero-boundary energy space Pointwise and energy difference estimates for odd powers

[F3] The time-integrability lemma applies to the stated compatible mixed-norm convention and establishes strong H01 measurability and L1 membership for the nonlinear terms, without assuming Bochner measurability into L∞. Cubic and quintic time bounds from L4 Linfinity control

[F4] Hölder's integral inequality applies to nonnegative measurable time functions, including its 1,∞ endpoint. Holder's inequality for integrals, including the endpoint cases

[F5] The assumed Axiom of Choice supplies the choice hypotheses carried by these inputs. The Axiom of Choice

Given: The domain, parameters, functions and norms in the Statement.

Goal: Establish Bochner integrability and both displayed estimates.

1.1givenF1F3F5

For w∈{u,v,v−u} put Aw(t)=∥w(t)∥L∞(Ω), aw=∥Aw∥L4(0,T) and Bw=sup⁡t∈[0,T]∥w(t)∥H1. The compatible representative convention makes Aw measurable and finite almost everywhere. The domain and norm hypotheses match [F3], so g(u) and g(v) are strongly measurable and integrable as H01-valued functions; their difference is integrable as well.

2.1F2F4step 1.1algebra

Apply [F4] to Avm−1 and 1 with reciprocal exponents (m−1)/4 and (5−m)/4. At m=5 the second exponent is ∞. Thus ∫0TAvm−1 dt≤T(5−m)/4avm−1. Using the sharp pointwise-in-time growth estimate in [F2] and av,Bv≤∥v∥XT gives the first displayed inequality.

3.1givenF2F4step 1.1step 2.1

Let h=v−u. The energy difference bound in [F2], applied at almost every time, is bounded by C[(Avm−1+Aum−1)Bh+(Avm−2+Aum−2)(Bv+Bu)Ah]. The first part integrates by step 2.1. Repeated Hölder applied to the second part gives ∫0TAwm−2Ah dt≤T(5−m)/4awm−2ah for w=u,v: the three reciprocal exponents are (m−2)/4, 1/4 and (5−m)/4, with the last factor in L∞ when m=5.

4.1step 3.1F1algebra∎

Bound each aw and Bw by ∥w∥XT. For nonnegative x,y, xm−2y+xym−2≤xm−1+ym−1, so the mixed terms from step 3.1 are absorbed into the displayed difference bound after enlarging C. The constant depends only on m and ∣λ∣, and (5−m)/4=0 for m=5.

Source notes

The original article is cited for attribution of the estimates in Proposition 3.9. The proof uses the separately supplied local polynomial and time-integrability results and Hölder's inequality.

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