REVIEW 2 major objections 5 minor 42 references
Model Order Reduction from Data with Certification
T0 review · 2 major / 5 minor · reviewed 2026-08-09 · deepseek-v4-flash
Pith's one-line read This paper claims that two recorded input-state trajectories of an unknown continuous-time linear system are enough to build a reduced-order model and a quadratic simulation function that formally bounds the difference between the reduced…
desk verdict New idea, flawed lemma: the data-driven simulation-function construction is attractive but the main theorem misses a rank condition on U0,T Q, so the certificate may be for the wrong system. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the quadratic simulation function $V(x,\hat{x}) = (x-\Theta\hat{x})^\top P(x-\Theta\hat{x})$, with $P \succ 0$ and $\Theta \in \mathbb{R}^{n \times \hat{n}}$. It works through three data-based identities: $I_n = X_{0,T}Q$ and $I_n = \bar{X}_{0,T}\bar{Q}$ express the drift matrix as $A = \bar{X}_{1,T}\bar{Q}$ and the pre-feedback matrix as $A+BF = X_{1,T}Q$, while $B$ is recovered as $(X_{1,T}Q-\bar{X}_{1,T}\bar{Q})(U_{0,T}Q)^\dagger$. The theorem's conditions then force the Lie derivative into the dissipativity inequality $\mathcal{L}V \le -\kappa V + \rho\|\hat{u}\|^2$: condition (3.8a) links the auxiliary matrix $H$ to $P^{-1}$, condition (3.8b) cancels the coupling terms between $x$ and $\hat{x}$, and condition (3.8c) makes the remaining quadratic term contractive. Because (3.8c) is equivalent to stabilizability of the pair $(A,B)$, the data-driven certificate inherits the classical necessary-and-sufficient condition of model-based design.
What would settle it
Pick a known stable linear system with two inputs and run the data-collection phase while applying control only through the first input, so the second column of $B$ is never excited; then follow Algorithm 1 and compare the certified bound from (2.5) with the actual sup-norm error $\|y(t)-\hat{y}(t)\|$ on a third trajectory driven through the second input. If the actual error exceeds the certified bound, the data-based representation (3.4) has not recovered the true $B$, and the simulation-function certificate fails.
Extended reading notes
Core claim
On its own terms, the paper's discovery is that the classical simulation-function framework for model order reduction does not require knowledge of $A$ and $B$; it requires only enough data to represent them. Lemma 3.2 shows that, under the rank condition that the stacked state matrices $X_{0,T}$ and $\bar{X}_{0,T}$ have full row rank, the closed-loop system under the interface map $u = U_{0,T}Q(x-\Theta\hat{x})+\Xi\hat{x}+\Psi\hat{u}$ has the data-based form (3.4), with $A$ recovered from the zero-input trajectory and $B$ recovered through the pseudoinverse expression $B = (X_{1,T}Q-\bar{X}_{1,T}\bar{Q})(U_{0,T}Q)^\dagger$. Theorem 3.4 then proves that the quadratic form $V(x,\hat{x}) = (x-\Theta\hat{x})^\top P(x-\Theta\hat{x})$ is a simulation function from the ROM to the unknown system whenever (3.8a)-(3.8c) hold, yielding the closeness guarantee $\|y(t)-\hat{y}(t)\| \le \frac{1}{\alpha}\beta(V(x,\hat{x}),t) + \frac{\rho}{\alpha\kappa}\|\hat{u}\|_\infty$. Because $\hat{C} = \Theta$ and the reduced-order model $\hat{A},\hat{B}$ is constructed entirely from this data, the guarantee applies to the actual unknown system rather than to an identified surrogate model.
Load-bearing premise
The load-bearing premise is that the two recorded trajectories reveal the full drift matrix $A$ (so $\bar{X}_{0,T}$ has full row rank) and that the control inputs used in the first trajectory excite every direction of $B$ that the interface map will use; without that, the data-based representation (3.4) can describe a different closed-loop system, and the certificate from Theorem 3.4 would not apply to the true unknown one.
Editorial extensions
If this is right
- A controller that enforces a safety, reachability, or reach-while-avoid specification on the data-driven ROM, refined through the interface map $u = U_{0,T}Q(x-\Theta\hat{x})+\Xi\hat{x}+\Psi\hat{u}$, guarantees the same specification on the unknown system up to the quantified bound (2.5).
- The construction needs only two recorded trajectories and no identification step: the rank conditions on $X_{0,T}$ and $\bar{X}_{0,T}$ are checkable from data, and the remaining matrices come from solving an LMI and the linear equation (3.8b).
- Choosing the ROM input matrix $\hat{B} = I_{\hat{n}}$ (or a scaled identity) makes the ROM fully actuated, and choosing $\Psi$ according to (3.13) minimizes $\rho$, shrinking the guaranteed error bound.
- Setting $x_0 = \Theta\hat{x}_0$ removes the transient term $\frac{1}{\alpha}\beta(V(x,\hat{x}),t)$ from (2.5), leaving the steady-state error $\frac{\rho}{\alpha\kappa}\|\hat{u}\|_\infty$.
- For autonomous (input-free) systems the verification problem simplifies: no interface map is needed, and conditions (3.8b)-(3.8c) reduce to $\bar{X}_{1,T}\bar{Q}\Theta = \Theta\hat{A}$ and the corresponding LMI with $\bar{X}_{1,T}$.
Reading between the lines
- Because condition (3.8c) is equivalent to stabilizability, the data-driven certificate inherits a diagnosable failure mode: for an unstabilizable system the LMI cannot be satisfied, so the method's pre-computation step already tells the user that no such certified ROM exists.
- The zero-input trajectory requirement implicitly demands that every mode of the drift matrix $A$ shows up in the autonomous data; when $A$ is singular, condition (3.2b) cannot be met and, as the paper's own limitation section concedes, $B$ must be known instead, narrowing the 'fully unknown' setting in that case.
- Following the paper's stated outlook, the same two-trajectory logic should extend to systems of the form $\dot{x} = AZ(x)+Bu$ by replacing $x$ with the nonlinear basis $Z(x)$, which would make the interface map nonlinear and would likely preserve the rank-and-LMI structure; testing this on a polynomial or trigonometric basis is a direct next step.
- The bound in (2.5) grows linearly with $\|\hat{u}\|_\infty$, so the practical tightness of the certificate depends on restricting the ROM input magnitude; pairing the method with bounded-input symbolic controllers would make the guaranteed error a design parameter rather than a post-hoc number.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a data-driven approach to model order reduction for unknown continuous-time linear control systems. From two collected input-state trajectories, it constructs a reduced-order model (ROM) and a quadratic simulation function, and uses the latter to provide a guaranteed bound on the output closeness between the unknown system and the ROM. The main result is Theorem 3.4, which relies on Lemma 3.2 to obtain a data-based closed-loop representation and then formulates LMI conditions from which the simulation function is derived. The paper also gives an algorithm and five numerical benchmarks where the ROM is used for controller synthesis enforcing safety, tracking, or reach-while-avoid specifications.
Significance. If the main theorem were correct, this would be a useful contribution to data-driven control and formal synthesis: it bypasses system identification, uses only two trajectories, and provides a certificate suitable for controller synthesis over unknown linear systems. The paper also has practical strengths: it gives an explicit algorithm, validates the approach on several benchmarks, and correctly invokes the standard simulation-function closeness result in Theorem 2.5. However, the certification statement is not established as written, because Lemma 3.2 omits a necessary rank condition and the proof of Theorem 3.4 uses an unjustified equality Q = HP. These are load-bearing issues for the central claim that the simulation function certifies the actual unknown system.
major comments (2)
- [Lemma 3.2, Eq. (3.4)] The derivation of the data-based closed-loop representation (3.4) is only valid if F = U0,T Q has full row rank, or if the interface inputs (Ξ\hat x + Ψ\hat u) always lie in the row space of F. From M := X1,T Q − \bar X1,T \bar Q = B U0,T Q, the paper concludes B = M(U0,T Q)†, which requires (U0,T Q)(U0,T Q)† = I_m. The hypotheses in Lemma 3.2 and Theorem 3.4 impose full-row-rank conditions only on X0,T and \bar X0,T (see Remark 3.3); no condition on U0,T Q is stated. If F is rank-deficient, for example because an input channel is identically zero during data collection, then B ≠ M F† on the complement of the row space of F, and the term B(Ξ\hat x + Ψ\hat u) in the true dynamics is replaced in (3.4) by M F†(Ξ\hat x + Ψ\hat u). The simulation function inequality is then certified for a different closed-loop system, not necessarily for the unknown ct-LCS. The authors should add the rank condition as an explicit assumption and explain how it can be checked from data, or alternatively restrict the interface map so that Ξ\hat x + Ψ\hat u lies in R(F⊤).
- [Theorem 3.4, proof after Eq. (3.9)] The proof states that 'since X0,T HP = In from (3.8a) and X0,T Q = In from (3.2a), one can conclude that Q = HP'. This implication is false when T > n, because X0,T has a non-trivial right nullspace and two different right inverses Q and HP can both satisfy X0,T Q = X0,T HP = In. The subsequent equality X1,T Q P^{-1} = X1,T H, and therefore the use of LMI (3.8c), depends on Q = HP. While Algorithm 1 sets Q = HP in Step 4, the theorem statement does not include this condition; it should be added explicitly, for example by quantifying over Q = HP or by adding the constraint Q = HP, and the proof should be revised accordingly.
minor comments (5)
- [Remark 3.5] The claim that (3.8c) is feasible if and only if (A,B) is stabilizable is not proved, and as stated it is questionable because (3.8c) is a data-dependent LMI whose interpretation in terms of (A,B) relies on the unstated identification Q = HP discussed above. The authors should either prove this equivalence or soften the remark.
- [Section 3.2, Limitations] The statement that the zero-input trajectory condition 'implies that matrix A of the system should be full rank' is not sufficient; for \bar X0,T to have full row rank, the pair (A, x0) must be such that the zero-input trajectory spans R^n. The limitation should be rephrased accordingly.
- [Eq. (3.13)] The proposed choice Ψ = Ψ1Ψ2 for minimizing ρ requires the matrix M^T P M to be invertible; this non-singularity condition is not stated.
- [Algorithm 1, Step 5] Fixing \hat A to be Hurwitz before solving (3.8b) may make (3.8b) infeasible; the authors should either solve for \hat A and Ξ simultaneously or state the range condition from Lemma 3.7 explicitly.
- [Section 4.4] The comparison with [Ion15] and [BBSC23] is not apples-to-apples because those methods target different ROM dimensions and different properties; a quantitative comparison at the same reduction order or on the same error metric would be more informative.
Circularity Check
No circularity: the simulation-function certificate is derived from an LMI and a dissipation proof, not fitted to the output-closeness bound; self-citations are contextual and not load-bearing.
full rationale
We found no circular step. The central construction is not self-referential: Theorem 3.4 fixes a quadratic V and proves the dissipation inequality (2.3b) algebraically from LMI (3.8) via Cauchy-Schwarz and Young inequalities, and the closeness guarantee (2.5) is then imported from the external result [ZA17, Thm 3.3]. The data enter only as an identification of A+BF and B through Lemma 3.2; neither V nor the bound is fitted to observed trajectory errors. In the benchmarks the certified bound is compared against actual errors, rather than being tuned to match them, so the claimed closeness is not forced by construction. The self-citations in the introduction and related work ([LSAZ22, LSMZ17, LSZ19, LSZ20, ZLZC23]) are contextual and not load-bearing; the load-bearing cited results [ZA17] have no author overlap with the present paper. The main weaknesses are correctness/assumption gaps rather than circularity: Lemma 3.2's step B = (X1,T Q − bar{X}_{1,T} bar{Q})(U0,T Q)^dag requires U0,T Q to have full row rank, a condition never stated, and Theorem 3.4's assertion Q = HP does not follow from (3.2a) and (3.8a) unless Q is chosen so. These gaps could invalidate the certificate for the actual unknown system, but they do not make the derivation equivalent to its inputs.
Assumptions & free parameters
free parameters (6)
- \hat n =
1 or 2 in benchmarks
- \hat\kappa =
3, 5, 1.5, 2 in benchmarks
- \varepsilon =
1 in benchmarks
- \hat A =
-1, -0.001, -0.002, -10^{-4} in benchmarks
- \hat B =
1 or 0.1 I in benchmarks
- Sampling time tau and input signal for first trajectory =
not reported
assumptions (5)
- domain assumption The unknown system is a continuous-time linear control system as in Definition 2.1.
- domain assumption Collected data is noise-free and derivatives are exactly known.
- ad hoc to paper X_{0,T} and \bar X_{0,T} are full row rank.
- domain assumption The pair (A,B) is stabilizable.
- ad hoc to paper The input data U_{0,T} Q has full row rank (or interface inputs lie in its row space).
Cite this review
Pith. "Pith review of Model Order Reduction from Data with Certification." pith.science (2026). https://pith.science/paper/C75QW57Y
@misc{pith2026250201094,
author = {Pith},
title = {Pith review of: Model Order Reduction from Data with Certification},
year = {2026},
howpublished = {\url{https://pith.science/paper/C75QW57Y}},
note = {Machine review of arXiv:2502.01094}
}
read the original abstract
Model order reduction (MOR) involves offering low-dimensional models that effectively approximate the behavior of complex high-order systems. Due to potential model complexities and computational costs, designing controllers for high-dimensional systems with complex behaviors can be challenging, rendering MOR a practical alternative to achieve results that closely resemble those of the original complex systems. To construct such effective reduced-order models (ROMs), existing literature generally necessitates precise knowledge of original systems, which is often unavailable in real-world scenarios. This paper introduces a data-driven scheme to construct ROMs of dynamical systems with unknown mathematical models. Our methodology leverages data and establishes similarity relations between output trajectories of unknown systems and their data-driven ROMs via the notion of simulation functions (SFs), capable of formally quantifying their closeness. To achieve this, under a rank condition readily fulfillable using data, we collect only two input-state trajectories from unknown systems to construct both ROMs and SFs, while offering correctness guarantees. We demonstrate that the proposed ROMs derived from data can be leveraged for controller synthesis endeavors while effectively ensuring high-level logic properties over unknown dynamical models. We showcase our data-driven findings across a range of benchmark scenarios involving various unknown physical systems, demonstrating the enforcement of diverse complex properties.
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