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Multi-Environment GLAMP: Approximate Message Passing for Transfer Learning with Applications to Lasso-based Estimators

T0 review · 3 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read Multi-environment GLAMP makes the exact risks of three high-dimensional transfer-learning Lasso estimators computable.

desk verdict The precise risk formulas for the three Lasso transfer estimators are new and mostly convincing, but the application theorems rely on unproved fixed-point existence and regularity conditions that need to be verified or weakened. read the letter →

arxiv 2505.22594 v2 pith:4IBYRFPQ submitted 2025-05-28 math.ST stat.MLstat.TH

classification math.STstat.MLstat.TH MSC 62J0762F1260F05
keywords Multi-EnvironmentGLAMPapproximatemessagepassingstateevolutiontransferlearninghigh-dimensionalregressionLassoriskdistributionshiftSecond-Stepestimator
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper introduces Multi-Environment Generalized Long AMP (GLAMP), an approximate-message-passing iteration that tracks several high-dimensional regression datasets at once when sample sizes and feature counts grow proportionally. The authors prove a state-evolution theorem: at every step, the GLAMP iterates in each environment behave like Gaussian matrices whose covariances are computed recursively, so any pseudo-Lipschitz functional of the iterates concentrates on its expectation. They then show that three Lasso-based transfer-learning estimators—the Stacked Lasso, the Model Averaging estimator, and the Second-Step estimator—are exactly tracked by this iteration. The payoff is precise asymptotic mean-squared-error formulas for these estimators, replacing worst-case minimax bounds with constants a practitioner can evaluate and compare under distribution shift.

What carries the argument

The load-bearing object is the multi-environment GLAMP iteration: alternating updates $V^t_e = \kappa_e^{-1}X_e^\top \Psi^{t-1}_e(R^{t-1}_e,W_e) - \kappa_e^{-1}\eta^{t-1}_e(V^{t-1}_1,\dots,V^{t-1}_E,B)[D^{(R)}]^\top$ and $R^t_e = X_e\eta^t_e(V^t_1,\dots,V^t_E,B) - \Psi^{t-1}_e(R^{t-1}_e,W_e)[D^{(V)}]^\top$, with Onsager terms equal to averaged Jacobians evaluated at Gaussian matrices. State evolution converts this recursion into a covariance recursion for the Gaussian limits. The estimator of interest is encoded by choosing the denoisers; for the Lasso cases, the fixed-design denoiser is the weighted Lasso map applied to Gaussian inputs. Distribution shift enters through environment-specific covariance matrices, noise levels, and environment weights, and the fixed-point systems close the recursion at the estimator of interest.

What would settle it

On a two-environment design-shift model with $\Sigma_2=\mathrm{diag}(\chi,1/\chi)$, numerically solve the fixed-point system (7) across a grid of $\lambda$ and $\chi$ and compare the predicted MSE with simulations at increasing $p$; a parameter point where the system has no solution or the solution branch jumps while simulated risk stays smooth would show Theorem 2 does not hold there. Equivalently, check whether the limit function $H$ in Assumption 4(b) is $C^1$ on the simulated region, since a failure of the derivative convergence already violates the theorem's hypotheses.

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Extended reading notes

Core claim

The paper's central claim is that in the proportional asymptotics limit, the risk of these transfer learners is governed by a fixed-design denoiser evaluated at the fixed point of state-evolution equations. Theorem 1 states that for any pseudo-Lipschitz function, the multi-environment GLAMP iterates converge to independent Gaussian matrices with covariances given by the state-evolution recursion. Theorems 2, 3, and 4 then assert that the Stacked Lasso, the Model Averaging estimator, and the Second-Step estimator converge to their fixed-design counterparts $\eta(\tau_1^*Z_1,\dots,\tau_E^*Z_E)$, $\sum_e \pi_e \eta_e(\tau_{\mathrm{ind},e}^* Z_e)$, and $\xi(\tau_{II}^*, \zeta^*, \eta(\tau^*))$, with parameters determined by the fixed-point systems (7), (15), and (23). Consequently, quantities such as $\mathbb{E}\|\eta(\tau^*) - \beta_1\|^2_{\Sigma}$ become exactly computable, and the simulated MSE curves track these formulas at moderate sample sizes.

Load-bearing premise

The paper's central claim stands on the existence of a solution to each fixed-point system (7), (15), and (23) and on $C^1$ convergence of the denoiser moment functions; if any system has no solution or has multiple solution branches, the asymptotic risk formulas are not justified.

Editorial extensions

If this is right

  • Exact MSE curves for the three estimators become computable expressions, so one can compare early fusion, late fusion, and pretrain-then-refine strategies without Monte Carlo.
  • The same state evolution gives the joint law of the estimators, not just marginal risk, so functionals such as support recovery or prediction error under a shifted test distribution are accessible.
  • Because the analysis handles anisotropic covariances and heterogeneous noise, it covers both covariate shift and model shift within one set of equations.
  • The framework's scope is not limited to the three estimators; changing the denoisers can yield precise risk characterizations for other transfer-learning penalties.
  • Cross-over behavior such as the simulated U-shaped MSE as a function of regularization suggests that adaptive tuning can be guided by the formulas.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Editorial extension: the fixed-design representation suggests the formulas may transfer to non-Gaussian covariate distributions via universality, but that requires a separate proof.
  • Editorial extension: comparing the stacked and second-step estimators implies an optimal mixture of source and target information; one could derive oracle environment weights from the fixed-point equations, a selection rule the paper does not spell out.
  • Editorial extension: iterating the second-step refinement could produce a multi-round transfer algorithm, but whether Assumptions 7 and 8 survive iteration is an open question.
  • Editorial extension: the assumption that noise and design are independent of the fixed signal sequences could be relaxed only if the Gaussian conditioning step is preserved; a small simulation study could test whether these formulas are robust to such relaxation.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper introduces Multi-Environment Generalized Long AMP (GLAMP), an approximate message passing framework for transfer learning with multiple Gaussian design matrices. Theorem 1 establishes a state evolution result for the multi-environment GLAMP iterates. The paper then applies this framework to three Lasso-based transfer learning estimators: the Stacked Lasso, the Model Averaging estimator, and the Second-Step estimator. For each, it derives an asymptotic risk characterization expressed through a fixed-design denoiser evaluated at a fixed point of the state evolution equations (Theorems 2, 3, and 4, respectively). The paper also reports finite-sample simulations that support the theoretical predictions.

Significance. If the stated results hold, this is the first exact risk characterization for these transfer learning estimators under proportional asymptotics, going beyond worst-case minimax bounds and complementing ridge-based random matrix analyses. The core theoretical contribution—a multi-environment GLAMP state evolution theorem—is a natural and potentially useful extension of the GLAMP framework, and the paper provides detailed appendices reducing the application theorems to the GLAMP convergence machinery. The paper also ships explicit, machine-checkable formulas for the asymptotic MSE of the three estimators, and the simulations suggest that the finite-sample predictions are accurate. The main weakness is that the application theorems are conditional on fixed-point existence and smooth-limit assumptions that are not verified for the Gaussian Lasso examples, so the strength of the final claims depends on resolving that gap.

major comments (3)
  1. [§4.1–§4.3, Assumptions 4, 6, 8, Eqs. (7), (15), (23)] Theorems 2–4 are stated as unconditional asymptotic characterizations, but they rest on Assumptions 4(a), 6(a), and 8(a), which merely assume existence of solutions to the fixed-point systems (7), (15), and (23), and on Assumptions 3(b,c), 4(b), 5(b,c), 6(b), 7, and 8(b), which assume C^1 convergence of the moment functions. The paper never verifies these conditions for the Gaussian Lasso transfer-learning model of Assumption 2, nor for the specific simulation settings of Figures 1–3. In particular, the proof of the Cauchy property (Lemma C.3) uses the contraction condition κ_e δ_e < 1, which is only guaranteed by the existence part of Assumption 4(a). If (7) has no solution, the limiting denoiser η(τ*,·) in Theorem 2 is not defined; if it has multiple branches, the GLAMP iterates could converge to a different branch and the risk formula would fail. The paper should either prove these fixed-point conditions for the Gaussian Lasso examples (e.g., by reducing to known scalar Lasso state-evolution results and a contraction argument for the multi-environment system), or explicitly present Theorems 2–4 as conditional and verify the assumptions numerically in the simulations, including reporting how (7), (15), and (23) are solved and checking κ_e δ_e < 1 and the derivative bounds.
  2. [Assumption 8(a) and Theorem 4] Assumption 8(a) is self-referential: it requires that the parameter γ_I in Equation (20) be chosen so that γ_I = γ_I, where γ_I is the limit of the Onsager coefficient γ_I^(p) from Proposition 3. The fixed-point system (23) does not include an equation determining γ_I; it treats γ_I as a free parameter and solves for τ_II*, ζ*, θ_II*. The condition γ_I = γ_I is therefore an extra fixed-point equation whose solvability is assumed, not established. As written, Theorem 4 characterizes only a second-step estimator whose penalty is tuned to an unknown asymptotic quantity, and the theorem gives no guidance for constructing such an estimator in finite samples. Please clarify the logical status of this condition and either prove existence of the self-consistent solution or reformulate Theorem 4 as conditional on it.
  3. [Appendix A.2, proof of Theorem 1] The sub-sequence argument in the proof of Theorem 1 asserts that after extracting a sub-sequence on which the cross-environment limits exist, the limiting covariance matrices used to characterize Z_{V,e}^t are 'fully determined by Assumption 1, independently of the choice of sub-sequences.' This is not immediate from the text, because the off-diagonal blocks of the full covariance matrix in the asymmetric GLAMP (Corollary A.1) can a priori depend on the extracted sub-sequence. The claim is, however, correct if one observes that the state evolution matrices Σ_{V}^t are block diagonal for t ≥ 1, since Ψ^t is block diagonal in Definition 3 and Definition 4; this directly yields the mutual independence of the Gaussian limits Z_{V,e}^t. Please add this argument explicitly to justify the independence claim and the sub-sequence independence.
minor comments (5)
  1. [§4.2, Theorem 3] In the first display of Theorem 3, the remainder is written as '+ o(1)', but the convergence is stochastic; elsewhere the paper uses '+ o_P(1)'. Please correct for consistency.
  2. [§4.1, simulation description] In the model-shift simulation, the text states that β̃_j iid ~ N(0, σ̃²π/2) satisfies ∥β̃∥_1/p → σ̃². The correct limit is σ̃, since E|β̃_j| = σ̃. Please fix this formula and the corresponding caption text.
  3. [Throughout] There are several typographical and notational slips, including 'Multi-enrivonment' in Section 1, 'satiesfy' in Assumption 4(b), 'diagnoal' in Appendices C and D, and the overloaded use of η in Equation (4) where the same symbol denotes both the argmin function and its value. These should be cleaned up.
  4. [§4.1, Eq. (2)] The notation β1,j = β2,j i.i.d.∼ N(0, σ_β²) in the simulation setup is ambiguous; presumably the intended meaning is β_e,j i.i.d. across both e and j, not that the two coordinates are identical. Please rephrase.
  5. [Assumption 8(b)] The definition of the family N(ε) and the condition 'for all i ≠ 2, with 1 ≤ i ≤ E+1, Cov(g_i, g_{i+E+1}) ≥ 1−ε' is quite involved and the role of ε_H is not discussed. A few sentences explaining the purpose of this assumption and how it is used would improve readability.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the state evolution equations and fixed-point systems self-consistently determine the asymptotic risk, and no fitted parameter is renamed as a prediction.

full rationale

The derivation chain is self-contained rather than circular. Theorem 1 is obtained by specializing the symmetric GLAMP state evolution of Gerbelot and Berthier [42], which is an external, independent result. Theorems 2, 3, and 4 then track the GLAMP iterates to the fixed-design denoisers η and ξ via Lemmas C.2, C.6, D.1, and E.7, with the fixed-point systems (7), (15), and (23) used only to determine the constants τ*, ϖ*, θ*, and related parameters. These systems are self-consistency conditions derived from the state evolution, not fits to simulated data: the paper repeatedly states that the theoretical MSE values come from the formulae (e.g., Figure 1 caption: 'The dotted line indicates the theoretical MSE value obtained from our formulae'), and the figures compare, rather than calibrate, those formulae to empirical values. The regularity and fixed-point existence assumptions (Assumptions 3(b),(c), 4(a),(b), 6(a),(b), 8(a),(b)) are conditions under which the theorems hold; they are not the conclusions renamed as assumptions. Any failure to verify these conditions for the simulated Gaussian examples is a technical gap or correctness risk, not circularity. The paper's self-citations (e.g., [25], [29], [30], [31], [61], [64], [71], [73]) appear in background and future-work contexts and are not load-bearing for the main convergence proofs, which rely on external AMP theory, Stein's lemma, and standard random matrix tools. No quoted reduction shows a fitted parameter being called a prediction, and no load-bearing argument reduces to a self-citation. Hence the circularity score is 0.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central derivation does not fit constants to simulated data. The risk formulas are evaluated at fixed points of state evolution equations derived from the model. The main uncharged inputs are the existing GLAMP theorem, the Gaussian-design domain assumption, and the assumed existence and regularity of fixed points. No new physical or probabilistic entities are postulated.

free parameters (1)
  • Fixed-point parameters in state evolution = Solutions of systems (7), (15), and (23); no closed form
    The risk formulas in Theorems 2 through 4 are evaluated at fixed points whose existence is assumed in Assumptions 4(a), 6(a), and 8(a). No primitive existence or uniqueness conditions are proven.
assumptions (4)
  • standard math The GLAMP convergence theorem of Gerbelot and Berthier [42] is used as a black box.
    Theorem 1 is proved in Appendix A by embedding the multi-environment GLAMP into the asymmetric GLAMP of [42] and applying a subsequence argument; the underlying convergence is cited, not reproved.
  • domain assumption The transfer learning model satisfies Assumption 2: linear model with Gaussian-design matrices X_e = X_iid_e Sigma_e^{1/2} and noise whose empirical distribution converges in 2-Wasserstein distance.
    This model is assumed throughout Section 4; the authors note that generalization to non-Gaussian designs is left for future work.
  • ad hoc to paper The denoiser moment functions E^(p) and H^(p) converge in C1 to their limits (Assumptions 3, 5, 7).
    These high-level conditions are stated to make the Onsager terms well-defined; the paper does not verify them for concrete Gaussian Lasso examples.
  • ad hoc to paper The fixed point systems (7), (15), and (23) have solutions, and the corresponding H maps satisfy contraction conditions via kappa_e delta_e < 1 and kappa_1 gamma_II < 1 (Assumptions 4, 6, 8).
    Theorems 2 through 4 evaluate risk at these assumed fixed points; uniqueness is argued via contraction constants, but no primitive verifiable conditions are supplied.

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Cite this review

Pith. "Pith review of Multi-Environment GLAMP: Approximate Message Passing for Transfer Learning with Applications to Lasso-based Estimators." pith.science (2026). https://pith.science/paper/4IBYRFPQ

@misc{pith2026250522594,
  author       = {Pith},
  title        = {Pith review of: Multi-Environment GLAMP: Approximate Message Passing for Transfer Learning with Applications to Lasso-based Estimators},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/4IBYRFPQ}},
  note         = {Machine review of arXiv:2505.22594}
}
read the original abstract

Approximate Message Passing (AMP) algorithms enable precise characterization of certain classes of random objects in the high-dimensional limit, and have found widespread applications in fields such as signal processing, statistics, and communications. In this work, we introduce Multi-Environment Generalized Long AMP, a novel AMP framework that applies to transfer learning problems with multiple data sources and distribution shifts. We rigorously establish state evolution for multi-environment GLAMP. We demonstrate the utility of this framework by precisely characterizing the risk of three Lasso-based transfer learning estimators for the first time: the Stacked Lasso, the Model Averaging Estimator, and the Second Step Estimator. We also demonstrate the remarkable finite sample accuracy of our theory via extensive simulations.

Figures

Figures reproduced from arXiv: 2505.22594 by the authors.

Figure 1
Figure 1. MSE of [PITH_FULL_IMAGE:figures/full_fig_p014_1.png] view at source ↗
Figure 2
Figure 2. MSE for βˆ average. Same settings as in [PITH_FULL_IMAGE:figures/full_fig_p018_2.png] view at source ↗
Figure 3
Figure 3. MSE for βˆ II. Same settings as in [PITH_FULL_IMAGE:figures/full_fig_p023_3.png] view at source ↗

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. On Universality of Non-Separable Approximate Message Passing Algorithms

    math.ST 2025-06 conditional novelty 8.0 of 10

    Non-separable AMP admits universal state evolution for non-Gaussian Wigner matrices when its nonlinearities are BCP-representable polynomials or BCP-approximable Lipschitz functions.

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    We plan to show ρt,t+1 e converges exponentially fast to 1 for each e ∈ [E] with an argument of fixed point iteration

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    Since ∆ η(2) = ∆ηt − ∆η(1), we know 1 p ∥diag(⃗λSc)(∆η(2))Sc∥1 − 1 p [diag(⃗λSc)st Sc]⊤(∆η(2))Sc ≤ ϵ2 · c2 2c4 3 + 4 √ 2ϵc2c3, where we have used the fact that M > 1 from (i)

    For ∆ η(1), we have 1 p [diag(⃗λSc)st Sc]⊤(∆η(1))Sc ≤ 1 p ∥diag(⃗λSc)(∆η(1))Sc∥1 ≤ 1√p ∥⃗λSc∥ ·1√p ∥∆ ˆβ(1)∥2 ≤ 2 √ 2ϵc2c3, where we have used Cauchy’s inequality and the bound on 1√p ∥⃗λ∥2 from (i) of the high-probability event. Since ∆ η(2) = ∆ηt − ∆η(1), we know 1 p ∥diag(⃗...

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    Σ 0 (ind,R,e) = " limp 1 p β⊤ e Σeβe limp 1 p E[β⊤ e Σeηe] limp 1 p E[β⊤ e Σeηe] lim p 1 p E[η⊤ e Σeηe] #

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    Simplifying it with Assumption 5 and 6, Σ1 (ind,V,e) = E[W 2 e ] + κe limp 1 p E[∥ηe − βe∥2 Σ(ind,e) ] = (τ ∗ ind,e)2

    Σ 1 (ind,V,e) = limp 1 ne E we + (Z0 (ind,R,e))1 − (Z0 (ind,R,e))2 2 2 where Z0 (ind,R,e) = [(Z0 (ind,R,e))1|(Z0 (ind,R,e))2] ∈ Rp×2 has iid rows drawn from N (0, κeΣ0 (ind,R,e)). Simplifying it with Assumption 5 and 6, Σ1 (ind,V,e) = E[W 2 e ] + κe limp 1 p E[∥ηe − βe∥2 Σ(ind...

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    Σ 1 (ind,R,e) =   limp 1 p β⊤ e Σeβe limp 1 p E[β⊤ e Σeηe] lim p 1 p E[β⊤ e Σeηe] limp 1 p E[β⊤ e Σeηe] lim p 1 p E[η⊤ e Σeηe] lim p 1 p E h η⊤ e,(1)Σeηe,(2) i limp 1 p E[β⊤ e Σeη] lim p 1 p E h η⊤ e,(1)Σeηe,(2) i limp 1 p E[η⊤ e Σeηe],   where ηe,(1) and ηe,(2) are mu...

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    For the diagnoal elements of Σ 2 (ind,V,e) we have simplified it in the same way as Σ 1 (ind,V,e)

    Σ 2 (ind,V,e) = " (τ ∗ ind,e)2 (τ ∗ ind,e)2Hind,e(0) (τ ∗ ind,e)2Hind,e(0) ( τ ∗ ind,e)2 # . For the diagnoal elements of Σ 2 (ind,V,e) we have simplified it in the same way as Σ 1 (ind,V,e). For the off-diagonal term, it is lim p 1 ne E h (we + (Z0 (ind,R,e))1 − (Z0 (ind,R,e)...

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    − κeδ (p) e (ye − rt−1 e ). Part II : v0 II = ζ ∗ · τ ∗ IIvt0 1 /τ ∗ 1 + p 1 − (ζ ∗)2 · τ ∗ IIZinit 1 , r0 II = X iid 1 Σ1/2 1 ξ v0 II, η(vt0 1 , · · ·, vt0 1 ) − (γIIτ ∗ IIζ ∗/τ ∗ 1 + γI) · κ1(y1 − rt0−1 1 ); ∀ t ∈ N+, vt II = X iid 1 ⊤ (y1 − rt−1 II − κ1γI(y1 − rt+t0−1 1 )) ...

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    We are left to verify that the state evolution is well-defined, and satisfies the marginal properties 68 in Lemma E.2

    = κ1 y1 − r0 1|y1 − r1 1| · · · |y1 − rt 1|0| · · · ∥, y1 − r0 II − κ1γI(y1 − rt0 1 )| · · · |y1 − rt−t0 II − κ1γI(y1 − rt 1)|0| · · · , ∀ e >1, Ψt e(Rt e) = κe ye − r0 e |ye − r1 e | · · · |ye − rt e|0| · · · . We are left to verify that the state evolution is well-defined, a...

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    71 By the law of iterated expectations, HII(1) ≥ HII(0)

    After some algebra, we have (τ ∗ 1 τ ∗ II)2HII(0) = (1 − κ1γI)2E[W 2 1 ]2 + κ1E[W 2 1 ] lim p 1 p E Σ1/2 1 β1 − ˘ξ − κ1γI(β1 − η) 2 2 + κ2 1 lim p 1 p E Σ1/2 1 β1 − ˘ξ − κ1γI(β1 − η) 2 2 lim p 1 p E Σ1/2 1 (β1 − η) 2 2 + κ1(1 − κ1γI)2E[W 2 1 ] lim p 1 p E Σ1/2 1 (β1 − η) 2 2 ,...

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    Then sII ∈ ∇µII(ξ; η), or in the case of the joint estimator, sII/λII ∈ ∂∥ξ − η∥1

    + (1 − κ1γI)eΣ1(β1 − η) − eΣ1(ξ − η) i . Then sII ∈ ∇µII(ξ; η), or in the case of the joint estimator, sII/λII ∈ ∂∥ξ − η∥1. Define eξt = ξt − ˆβstack+ eθ∗ IIst II/λII. Recall we have defined St II(c) = {j ∈ [p] : |(st II)j| ≥λII(1−c)}. Since st II/λII ∈ ∂∥ξt − ˆβstack∥1, St II...

  92. [100]

    We write ( ¯˜ξ)j = hII,j(Z′ 1,j), where hII,j(·) also implicitly depends on Z1, · · ·, ZE and P ⊥ 1,jZ′ 1, P ⊥ 1,j being the projection matrix onto the orthogonal complement of span( eσ1,j). Note that these other Gaussian variables hII,j(·) implicitly depends on are all indepe...

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    (E-38) We are left to bound1 p ∥∆ξ(2)∥2

  94. [102]

    1 + ∥X∥2 2 N k−1 + ∥Y ∥2 2 N k−1# ≤ 3qk−1

    We utilize the first line of Eqn (E-37). For ∆ξ(1), 1 p [(st II)Sc II ]⊤(∆ξ(1))Sc II ≤ 80 1 p ∥(∆ξ(1))Sc II ∥1 ≤ 1√p ∥∆ξ(1)∥2 ≤ 2 √ 2ϵc2c3. Since ∆ ξ(2) = ∆ξt − ∆ξ(1), we know 1 p ∥(∆ξ(2))Sc II ∥1 − 1 p [(st II)Sc]⊤(∆ξ(2))Sc II ≤ ϵ2 · (c2c3)2/λII + 4 √ 2ϵc2c3. Recall that St I...

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Reviewed August 7, 2026 · model on record in the stance chip above.