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Bounds for the truncated logarithm: ∣LN(h)zi/h∣≤8M/3 and ∣Xj∣≤(8M/3)(C0Mj/ρ)j−1

Statement

Let d≥1, let (U,Uρ,A~,B~,M,K,δ,Ψh) be analytic splitting data, and use the notation of The truncated binomial family Ths,N, the truncated logarithm LN(h), its coefficient fields Xj and the truncations X[N] and The difference operator Δh=Ψh∗−id loses a controlled domain margin and is bounded by CdM∣h∣/b: b=ρ/(8N), rN=ρ/(C0MN) with C0=max⁡{16,128d2}, the domains Dk, the difference operator Δh, the truncated logarithm LN(h) and the coefficient fields Xj, 1≤j≤N. Then for every integer N≥2, every ∣h∣≤rN and every coordinate index i:

  1. ∥Δhkzi∥Dk≤2M∣h∣ qk−1 for 1≤k≤N, where q=CdM∣h∣/b≤1/4.

  2. The function h↦(LN(h)zi)(Z)h, initially defined for 0<∣h∣≤rN, extends holomorphically to the disc ∣h∣≤rN, and ∣(LN(h)zi)(Z)h∣≤2M1−q≤8M3(∣h∣≤rN, Z∈DN).

  3. For 1≤j≤N and every Z∈DN, max⁡i∣(Xj)i(Z)∣≤8M3 rN−(j−1), and the sharper estimate max⁡i∣(Xj)i(Z)∣≤8M3(C0Mjρ)j−1, obtained for j≥2 by computing the j-th field with truncation order j, and for j=1 from X1=XA~+XB~; the two bounds agree in the form rN−(j−1)=(C0MN/ρ)j−1. The corresponding Euclidean field bounds are these component bounds multiplied by 2d.

  4. The field Xj is independent of the truncation order: if N,N′≥j then the j-th coefficient fields of LN and LN′ agree on D(ρ/4).

Facts & Assumptions

Given: The analytic splitting data, N≥2, b=ρ/(8N), rN=ρ/(C0MN), the fields Xj and functions LN(h)zi of The truncated binomial family Ths,N, the truncated logarithm LN(h), its coefficient fields Xj and the truncations X[N].

[L1]

Difference-operator bounds (The difference operator Δh=Ψh∗−id loses a controlled domain margin and is bounded by CdM∣h∣/b): ∥Ψh(Z)−Z∥≤2M∣h∣≤b on Dk, and for f holomorphic on Dk−1 with ∣f∣≤F, ∣Δhf∣≤qF on Dk with q=CdM∣h∣/b≤1/4; hence ∣Δhkf∣≤qkF on Dk.

[L2]

Cauchy coefficient estimates in one variable (Cauchy's inequalities bound the Taylor coefficients by the circle supremum): if G is holomorphic on D(0,R) and ∣G∣≤A on ∣h∣=r<R, then the j-th Taylor coefficient obeys ∣cj∣≤A/rj.

[L4]

Removable singularities and coefficient extraction: a function holomorphic and bounded on a punctured disc extends holomorphically to the centre, and the coefficients coincide with the Cauchy coefficients of Calculus of holomorphic maps: chain rule, Wirtinger derivatives, coefficient extraction in one variable, and locally uniform limits clause 3; the truncated logarithm is a finite linear combination of the Δhkzi, whose lowest h-degree in the k-th term is k, so (LN(h)zi)(Z)=O(h) at h=0 (Iterated Cauchy formula, power-series expansion and Cauchy estimates for functions continuous and separately holomorphic on a polydisc, Calculus of holomorphic maps: chain rule, Wirtinger derivatives, coefficient extraction in one variable, and locally uniform limits).

[L5]

Taylor coefficients of LN versus LN′: LN−LN′=∑k=min⁡+1max⁡(−1)k+1kΔhk is a sum of terms of h-degree ≥min⁡(N,N′)+1 (The truncated binomial family Ths,N, the truncated logarithm LN(h), its coefficient fields Xj and the truncations X[N]).

Proof

technique · direct; combine the displacement bound with the single-step operator bound, sum the geometric series, and apply Cauchy's coefficient estimate in $h$
1.1givenL1algebra

Iterate bound. For Z∈D1, ∣Δhzi(Z)∣=∣zi(Ψh(Z))−zi(Z)∣≤∥Ψh(Z)−Z∥≤2M∣h∣ by [L1]; applying the operator bound [L1] a further k−1 times gives ∣Δhkzi(Z)∣≤qk−1⋅2M∣h∣ on Dk. This is clause 1.

2.1givenstep 1.1L3L4algebra

Bound for the truncated logarithm. On DN, ∣LN(h)zi(Z)h∣≤∑k=1N∣Δhkzi(Z)∣k ∣h∣≤2M∑k=1Nqk−1k≤2M1−q≤8M3, using q≤1/4. The quantity (LN(h)zi)(Z) is O(h) at h=0 by [L4], and h↦(LN(h)zi)(Z)/h is holomorphic for 0<∣h∣≤rN by [L3]; the bound just proved makes h=0 a removable singularity, so the extension is holomorphic on ∣h∣≤rN. This is clause 2.

3.1givenstep 2.1L2algebra

Coefficient bound. Fix Z∈DN and let G(h):=(LN(h)zi)(Z)/h denote the holomorphic extension of clause 2. It is bounded by 8M/3 throughout the open disc ∣h∣<rN. For any radius 0<r<rN, the Taylor coefficient of order j−1, namely (Xj)i(Z) (The truncated binomial family Ths,N, the truncated logarithm LN(h), its coefficient fields Xj and the truncations X[N]), obeys ∣(Xj)i(Z)∣≤(8M/3)r−(j−1) by [L2], whose circle hypothesis is strict. Letting r↑rN gives ∣(Xj)i(Z)∣≤(8M/3)rN−(j−1). Taking the maximum over i gives the first estimate. The finite difference LN−LN′ starts at degree min⁡(N,N′)+1 by [L5], so coefficients through that degree minus one coincide. For j≥2 we may therefore use truncation order j, giving the sharper estimate. For j=1, differentiating the three subflows at zero gives X1=XA~+XB~, with each coordinate bounded by 2M≤8M/3.

4.1givenL5algebra

Independence of the truncation order. If N,N′≥j, then by [L5] the difference LN(h)zi−LN′(h)zi has no terms of h-degree ≤min⁡(N,N′), hence none of degree ≤j; dividing by h and extracting the coefficient of hj−1 gives the same value for both truncation orders, on the common domain D(ρ/4). This is clause 4, and it justifies the substitution made in step 3.1.

5.1step 1.1step 2.1step 3.1step 4.1∎

Conclusion. Clauses 1, 2 and 4 are steps 1.1, 2.1 and 4.1; clause 3 is step 3.1. All constants depend only on d, M and ρ.

Remarks

  • Note the displayed first estimate of clause 3 contains a redundant form; the usable statement is max⁡i∣(Xj)i(Z)∣≤(8M/3)(C0Mj/ρ)j−1, which exhibits the factorial-type growth of the modified-field coefficients and is the input to the truncation choice of Optimal finite truncation gives an exponential local splitting defect. The bound is uniform on DN and depends on M, ρ and d only.
  • No convergent infinite series is used: LN is a finite sum and the coefficients are extracted with Cauchy's formula in the single complex variable h.

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