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The linear homological equation ν⋅∂ϑu=v on a Diophantine label set, with uniform parameter bounds and conservative loss

Statement

Let n≥1, τ>n−1, α>0, and let Ωα,τ be the label set of Analytic KAM data: angular tori, action-angle neighbourhoods, Fourier cutoffs and Diophantine frequency labels. Let r,s>0, K≥0, and suppose one of the following periodic, zero-mean data conditions holds.

  • (G) General case: for each real ν∈Ωα,τ, v(⋅,⋅,ν) is holomorphic on Br×Ss, with N:=sup⁡ν∈Ωα,τsup⁡I∈Br, ϑ∈Ss∣v(I,ϑ,ν)∣<∞. This is a family of phase-holomorphic functions; no holomorphy on a complex parameter neighbourhood is assumed.
  • (T) Truncated case: h>0, and v is jointly holomorphic on Dr,s,h(Ωα,τ), with N:=∣v∣r,s,h<∞ and vk=0 for ∣k∣1>K. If K>0, assume h≤α/(2Kτ+1). If K=0, the zero-mean datum is identically zero and no divisor restriction on h is needed.

Put α0:=α in (G) and α0:=α/2 in (T). In (G), for a family w of phase functions write ∥w∥a,bG:=sup⁡ν∈Ωα,τsup⁡I∈Ba, ϑ∈Sb∣w(I,ϑ,ν)∣; this is explicitly a supremum of phase-space suprema over real labels. In (T), norms retain the joint convention ∣⋅∣a,b,h.

For every parameter of the relevant case and every nonzero k in the Fourier support of v, ∣k⋅ν∣≥α0∣k∣1−τ. For nonzero k, define ak:=vk/(ik⋅ν) in (G), and for 0<∣k∣1≤K in (T); set ak:=0 for ∣k∣1>K in (T). The series u(I,ϑ,ν):=∑k∈Zn, k≠0ak(I,ν)eik⋅ϑ converges absolutely and uniformly on Br×Ss−2σ at every fixed label in (G), uniformly over those real labels, and on Dr,s−2σ,h(Ωα,τ) in (T), whenever 0<σ≤1 and 2σ<s. Its sum is phase-holomorphic at each label in (G), jointly holomorphic in (T), periodic, zero-mean, and satisfies ν⋅∂ϑu=v. Any two periodic holomorphic zero-mean solutions on a common such case domain agree there.

For every 0<σ≤1 with 4σ<s, every i,j∈{1,…,n}, and every 0<δ<r, the following bounds hold in the joint case (T): ∣u∣r,s−2σ,h≤Cn,τα0σ−(τ+n)N,∣∂ϑju∣r,s−3σ,h≤Cn,τα0σ−(τ+n+1)N, ∣∂Iju∣r−δ,s−2σ,h≤Cn,τα0 δσ−(τ+n)N,∣∂ϑi∂ϑju∣r,s−4σ,h≤Cn,τα0σ−(τ+n+2)N, ∣∂ϑi∂Iju∣r−δ,s−3σ,h≤Cn,τα0 δσ−(τ+n+1)N,∣∂Ii∂Iju∣r−δ,s−2σ,h≤Cn,τα0 δ2σ−(τ+n)N. In (G), the same six inequalities hold with each joint norm ∣w∣a,b,h replaced by the explicit supremum ∥w∥a,bG; thus no output bound is a norm on the empty set O0(Ωα,τ).

In (T), uk=0 for ∣k∣1>K; in particular u and its displayed derivatives have the same Fourier support as v.

Facts & Assumptions

Given: The dimension n, exponent τ>n−1, constant α>0, label set Ωα,τ, widths r,s and (in (T)) h>0, cutoff K, and the periodic zero-mean data of case (G) or (T), with the bound N defined in the corresponding case above.

[F1]

The definition of Dr,s,h, holomorphy, Fourier coefficients, angular mean, Fourier cutoff, and the (α,τ)-Diophantine condition ∣k⋅ν∣≥α∣k∣1−τ for every nonzero k (Analytic KAM data: angular tori, action-angle neighbourhoods, Fourier cutoffs and Diophantine frequency labels).

[F2]

In (G), for each fixed label the phase Fourier series of v equals v on each smaller strip, its coefficients obey ∣vk∣≤Ne−(s−ρ)∣k∣1 for 0<ρ<s, and it can be differentiated termwise locally uniformly. In (T), the same assertions hold jointly on smaller action, angle and parameter domains (Fourier decay, Cauchy derivative bounds, lattice shell counts and the exponential small-divisor sums on action-angle neighbourhoods clauses 1 and 2).

[F3]

The lattice sums include ∑k≠0∣k∣1τe−σ∣k∣1≤Cn,τσ−(τ+n) for 0<σ≤1, and the corresponding sums with one or two extra powers of ∣k∣1 (Fourier decay, Cauchy derivative bounds, lattice shell counts and the exponential small-divisor sums on action-angle neighbourhoods clause 3).

Proof

Given: In (T), every quotient is formed only for retained modes 0<∣k∣1≤K; discarded coefficients are zero. In (G), each fixed parameter is a real Diophantine label and all nonzero modes are retained.

Proof technique: direct; estimate the divisors, sum the phase Fourier series, and use coordinate Cauchy estimates for its action derivatives.

1.1givenF1algebra

Small divisors. In (G), ∣k⋅ν∣≥α∣k∣1−τ for each [given, F1, algebra] real label and every nonzero mode. In (T), fix ν∈Oh(Ωα,τ) and a retained mode 0<∣k∣1≤K. There is a real label ν0 with ∣ν−ν0∣∞<h, so ∣k⋅ν∣≥α∣k∣1−τ−∣k∣1h≥(α/2)∣k∣1−τ, since h∣k∣1τ+1≤hKτ+1≤α/2. No discarded mode is divided by its divisor.

2.1step 1.1F2F3F4algebra

Convergence and holomorphy. Let 0<σ≤1 with 2σ<s. For [step 1.1, F2, F3, F4, algebra] each label in (G), and uniformly over those labels by the definition of N, or on the joint domain in (T), [F2] and step 1.1 give ∑k≠0∣vkik⋅νeik⋅ϑ∣≤Nα0∑k≠0∣k∣1τe−σ∣k∣1≤Cn,τα0σ−(τ+n)N by [F3]. Thus the series is absolutely and uniformly convergent on the specified phase strip (jointly in all variables in (T)); it is holomorphic in the phase variables in (G) and jointly holomorphic in (T), by [F4]. Each term has zero angular mean, hence [u]=0. Taking the appropriate supremum gives the first bound in clause 2, using ∥⋅∥G in (G).

3.1step 2.1F2F3F4algebra

The equation and angular derivatives. On every strictly smaller strip, [step 2.1, F2, F3, F4, algebra] the differentiated series is locally uniformly convergent by [F2] and [F3]. Termwise differentiation yields ∂ϑju=∑k≠0vkkjk⋅νeik⋅ϑ,ν⋅∂ϑu=∑k≠0vkeik⋅ϑ=v−[v]=v, where the last equality follows from [F2]. The extra factor ∣kj∣≤∣k∣1 and the two angle margins in clause 2 give the stated one- and two-angle bounds using [F3].

3.2step 1.1step 2.1F2F3algebra

Action derivatives. By [F2], vk is holomorphic in the action on Br [step 1.1, step 2.1, F2, F3, algebra] and ∣vk∣≤Ne−(s−ρ)∣k∣1. The coordinate Cauchy estimates of Fourier decay, Cauchy derivative bounds, lattice shell counts and the exponential small-divisor sums on action-angle neighbourhoods clause 2, with action margin δ, give on Br−δ ∣∂Ijvk∣≤Nδe−(s−ρ)∣k∣1,∣∂Ii∂Ijvk∣≤2Nδ2e−(s−ρ)∣k∣1. These estimates are pointwise at every label in (G), uniformly by N, and jointly on the smaller parameter domain in (T). Division by the divisor and summation using [F3] prove the action and mixed action-angle bounds in clause 2, with the explicit label supremum in (G). The locally uniform convergence also justifies termwise differentiation.

3.3givenstep 2.1algebra

Truncation. In (T), vk=0 for ∣k∣1>K and the solution coefficient [given, step 2.1, algebra] is defined to be zero there, so every displayed derivative has support in ∣k∣1≤K.

4.1step 1.1step 2.1step 3.1F2algebra∎

Uniqueness. For a fixed real label in (G), Fourier coefficients of any [step 1.1, step 2.1, step 3.1, F2, algebra] zero-mean phase-holomorphic solution w obey (ik⋅ν)wk=vk for every nonzero mode, so every coefficient agrees with u. In (T), the same comparison applies on the open complex parameter domain. For retained modes, equality follows from step 1.1. For a discarded mode, vk=0 and (k⋅ν)wk=0; away from the hyperplane k⋅ν=0, wk=0, and continuity extends this across that hyperplane because its complement is dense in each open ball. The zero coefficient vanishes by the zero-mean condition. Fourier uniqueness from [F2] gives equality on the common domain in either case.

Remarks

  • What "truncated" refers to. Case (T) is the situation in the KAM step: the unknown is solved only for the Fourier-truncated data, and the parameter margin h has to be small enough that the Diophantine bound survives on the complex parameter neighbourhood, at the price of replacing α by α/2. No loss in α occurs in the parameter-free case (G).
  • Conservatism. The exponents are exactly those proved by summing the lattice shells; the number τ+n+2 appearing in the second derivatives is the largest loss used downstream, and no sharper small-divisor theorem is invoked.

Depends on

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