REVIEW 2 major objections 4 minor 34 references
Joint Communication-Control Strategy Optimization with Partially Nested Information Structures: The Linear-Quadratic Case
T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read For partially nested multi-agent linear-quadratic systems, optimal control stays linear under every fixed open-loop communication schedule, and the paper gives closed-form Riccati recursions to compute it.
desk verdict Credible Riccati-solvable DP for PN JCCO; the restrictive assumptions are honestly scoped and supported by counterexamples. 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 load-bearing object is the strict expansion of the information structure: starting from the fixed-schedule problem, add to the common information every past control action that has a nonzero input matrix and is already known to a recipient whose information is common, and remove those actions from private information. This expansion turns a partially nested information structure into a strictly partially nested one and makes the common-information-based belief strategy-independent, so the belief is a Gaussian whose mean is a fixed linear function of the common record and whose covariance is deterministic (Lemma IV.3). The Gaussian mean then serves as the state of a Markovian dynamic program, and optimizing over strategies that are linear in the mean and private information reduces to a centralized Kalman filter for the mean and the Riccati recursions of Theorem IV.6 for the gains. The paper also uses the finite set of open-loop schedules to close the loop: enumerate schedules, run the recursion for each, add communication costs, and pick the minimum.
What would settle it
Build the two-agent, two-step scalar system in Lemma III.5: agent 1 acts first, agent 2 observes only a noisy version of the state after agent 1's action, and agent 1's action influences the state but agent 2's observation matrix is zero for that influence. If communication of agent 1's private observation is allowed at zero communication cost, then a team-optimal strategy must be nonlinear; verifying numerically that the optimal cost over linear strategies is strictly worse than the optimal nonlinear strategy settles whether Assumption III.4 is needed. Equivalently, one could compute the Riccati value under Assumption III.4 on random matrices and check that it coincides with the value of an explicit nonlinear search in small horizons.
Extended reading notes
Core claim
Let a joint communication-control optimization (JCCO) problem be partially nested, with information evolution satisfying Assumption II.1, useless controls excluded by Assumption III.2, and every effective control observable by at least one other agent at the next time step by Assumption III.4. The paper proves that for any fixed open-loop communication strategy, the induced problem is a decentralized LQG problem with a partially nested information structure, and that a team-optimal control strategy exists that is linear in the available information (Theorem III.6). The proof route expands the information structure so that actions influencing other agents' information are moved into common information; in the expanded problem the common-information-based belief is strategy-independent and Gaussian, so its conditional mean and covariance are finite-dimensional sufficient statistics. A backward Riccati recursion (Theorem IV.6), fed by a centralized Kalman filter for the mean and a convex quadratic minimization for the private gains, computes the optimal linear strategy for each fixed schedule; minimizing over the finite set of schedules solves the original JCCO problem. With closed-loop communication, assuming communication strategies depend only on common information and past messages (Assumption V.1) plus an attainment condition stated in Appendix D-B, the same expansion yields a dynamic program over finite-dimensional Gaussian beliefs.
Load-bearing premise
Assumption III.4: every control action that actually affects the state must show up in at least one other agent's observation at the next time step; the paper demonstrates that without this assumption a partially nested problem can have only nonlinear optimal control strategies, so the linear closed-form solution collapses.
Editorial extensions
If this is right
- For every fixed open-loop communication schedule, the optimal control strategy is linear and is computed by the closed-form Riccati recursions; the globally optimal schedule is found by finite enumeration.
- The assumptions III.2 and III.4 are not technical decorations: dropping either can force nonlinearity or non-existence of the team-optimal control strategy, as shown by explicit two-agent two-step counterexamples.
- As a byproduct, the same recursions solve decentralized LQG control with partially nested information structures and output feedback under the common-information-based approach, including settings with singular noise covariances and positive-semidefinite cost matrices.
- Under closed-loop communication with Assumption V.1, the dynamic program runs over finite-dimensional Gaussian conditional means and covariances, so the infinite-dimensional belief state of a direct common-information treatment is avoided.
Reading between the lines
- Editorial extension: The schedule enumeration is exponential in the number of agents and horizon; a natural next step is branch-and-bound or beam search over schedules using the Riccati value as a lower bound, which the paper does not investigate.
- Editorial extension: Assumption III.4 is a one-step observability condition; one could test whether it can be replaced by a global detectability condition on the pair formed by the state transition and observation maps, which would extend the method to systems where influence takes two or more steps to reach another agent's observations.
- Editorial extension: The linearity result suggests a concrete test on random instances: compare the Riccati-computed policy against a nonlinear policy found by numerical search; if the assumptions hold, the linear policy should match or beat it, and violations of III.4 should show a gap.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper formalizes a joint communication-control strategy optimization (JCCO) problem for multi-agent LQG systems under the common-information-based (CIB) framework. For open-loop communication strategies, it gives structural conditions (Assumptions II.1, III.2, III.4) under which additional sharing preserves partial nestedness and a linear optimal controller exists (Theorem III.6). It then constructs a strict expansion that satisfies the SI-CIB condition, shows the CIB belief is Gaussian, and derives Riccati recursions for the optimal controller (Theorems IV.2, IV.4, IV.6). The same machinery is extended to closed-loop communication strategies under Assumption V.1, yielding a finite-dimensional dynamic program (Section V, Algorithm 1). The paper includes counterexamples showing necessity of the structural assumptions and a numerical study.
Significance. If the main results are correct, the paper is a significant contribution: it connects partial nestedness with the strategy-independent CIB condition and provides a candidate closed-form solution for a class of decentralized LQG problems with output feedback and communication optimization. The appendix contains detailed proofs and explicit counterexamples for the necessity of Assumptions III.2 and III.4, which is a strength. The numerical experiments illustrate the method on four information structures. However, the correctness of the central Riccati recursion depends on a Kalman-filter step that I believe is not justified; this must be repaired before the paper's main computational claim can be accepted.
major comments (2)
- [§C-6, Eq. (C.6); Theorem IV.6] The recursion for eΘ_{h+1} is derived by the standard known-input Kalman update, but eU_h is not an exogenous input: by Lemma IV.5 it equals bE_h eΘ_h + F_h Ip,h(eS_h-eΘ_h), hence it is correlated with the filter error eS_h-eΘ_h given eC_h. Strict partial nestedness only ensures eU_h (and possibly the relevant private labels) belong to eC_{h+1}; it does not make the prediction E[eS_{h+1}|eC_h,eU_h] equal to A_h eΘ_h+B_h eU_h. A minimal Gaussian counterexample to this formula is X_1∼N(0,1), U_1=X_1, X_2=X_1+U_1+W, Y=[U_1, X_2+V]^T with unit variances: the exact conditional mean of X_2 given Y is 2U_1+0.5(Y_2-U_1), whereas the paper's update gives U_1+0.5(Y_2-U_1). Consequently the matrices {K^j_{h+1}} and the Riccati recursions (IV.5)-(IV.7) are not established; this is load-bearing for the claimed closed-form solution.
- [Section V, Appendix D-B (Algorithm 1)] The closed-loop DP is presented under the unproved assumption that all displayed Bellman minima are attained by admissible strategies. Since the strategy spaces over continuous states are not compact and the joint minimization over communication actions and control prescriptions can have infima that are not attained, this assumption is not automatic. The paper should either prove attainment under the stated hypotheses or reformulate the claim in terms of epsilon-optimal strategies.
minor comments (4)
- [Eq. (IV.1)] The condition for adding \bar U_{i,t} to \bar C_h uses \bar I_{i,t}\subseteq \bar C_h, but \bar I_{i,t} is not explicitly defined for the fixed problem D(g^m_{1:H}) after the bar notation is introduced; please define it in the reduced notation.
- [Section II-A] The notation for the additional-sharing information is inconsistent: Z_h^a, Z^a_h, and \cup_i Z_{i,h}^a are used interchangeably.
- [Section VI-B, Figures 1 and 2] The line labels 'N' in Figure 2 should be defined in the caption, and it would be useful to report standard deviations over the 10 random seeds in Figures 1 and 2.
- [Theorem IV.6] The definitions eL^1_h=eL^3_h and eL^2_h=eL^4_h are duplicated; a single definition would reduce confusion.
Circularity Check
No significant circularity: the open-loop derivation is self-contained and the Riccati recursions follow from stated assumptions plus external PN/CIB results.
full rationale
The paper's central claim is not circular. For fixed open-loop strategies, Theorem III.6 proves the induced problem D(gm) is PN using Assumptions II.1, III.2, and III.4, and linear optimality then follows from Corollary A.2, which is proved in the appendix from the external Ho-Chu PN team theorem [15]. The strict expansion in Equation (IV.1) is an explicit construction, and Theorem IV.2 proves, rather than assumes, that the expanded problem is strictly PN and satisfies the SI-CIB condition; the proof uses only the system assumptions and partial nestedness. Lemma IV.3 is adapted from the external SI-CIB result [11], and its Gaussian-belief conclusion is conditional on the SI-CIB property proved in Theorem IV.2. Theorem IV.4 is a direct cost-equivalence calculation. Lemma IV.5 establishes linear CIB-Markovian optimality by an L2 projection argument over all admissible prescriptions, with reachable-mean quadratic value functions; it does not fit a linear ansatz and then relabel it as optimal. Theorem IV.6 derives the Riccati recursions from a standard Kalman-filter conditional-mean update and convex quadratic minimizations; no fitted parameter is renamed as a prediction. The self-citations to the authors' prior [14] are provenance for the strict-expansion technique and Assumption V.1, not load-bearing: the closed-loop extension is proved in the appendix via Lemma D.2 and Theorem D.3, and the paper explicitly discloses the attainment assumption ('We assume that all displayed Bellman minima have choices defining admissible strategies', Appendix D-B), which is a limitation rather than a circular step. No step in the derivation reduces to its own inputs by construction.
Assumptions & free parameters
assumptions (7)
- domain assumption The induced decentralized LQG problem ˇD is partially nested.
- domain assumption Information evolution follows fixed projection functions (Assumption II.1).
- domain assumption Zero-input actions do not appear in later information (Assumption III.2).
- domain assumption Every state-affecting action is observed by another agent at the next step (Assumption III.4).
- domain assumption Closed-loop communication strategies depend only on common information and past communication actions (Assumption V.1).
- ad hoc to paper Bellman minima in Algorithm 1 are attained by admissible strategies.
- standard math Singular Gaussian conditioning with Moore-Penrose pseudo-inverses is valid.
Cite this review
Pith. "Pith review of Joint Communication-Control Strategy Optimization with Partially Nested Information Structures: The Linear-Quadratic Case." pith.science (2026). https://pith.science/paper/AHNCSRRW
@misc{pith2026260813535,
author = {Pith},
title = {Pith review of: Joint Communication-Control Strategy Optimization with Partially Nested Information Structures: The Linear-Quadratic Case},
year = {2026},
howpublished = {\url{https://pith.science/paper/AHNCSRRW}},
note = {Machine review of arXiv:2608.13535}
}
read the original abstract
In this paper, we formalize a joint communication-control strategy optimization (JCCO) problem in multi-agent linear systems with quadratic costs, under the common-information-based (CIB) framework from decentralized stochastic control. For computational tractability, we focus on such JCCO problems with partially nested (PN) information structures (ISs). In particular, with a baseline communication protocol that leads to a PN IS, we establish a series of conditions under which the partial nestedness is preserved under the (additional) communication strategies to be optimized, while violating them may cause nonlinearity of the optimal strategies in general, with open-loop communication strategies. We then develop a dynamic-programming-based approach to compute the optimal control strategies of JCCO with open-loop communication strategies, which yields a set of closed-form Riccati Equations. As a byproduct of independent interest, such an approach also offers a way to solve decentralized linear-quadratic control with PN ISs and output feedback, under the CIB framework. Finally, we extend such an approach to JCCOs with closed-loop communication strategies, yielding a more tractable dynamic program than an infinite-dimensional CIB-belief-based one.
Figures
Reference graph
Works this paper leans on
-
[17]
A. Mahajan and A. Nayyar, “Sufficient statistics for linear control strategies in decentralized systems with partial history sharing,”IEEE Trans. Autom. Control, vol. 60, no. 8, pp. 2046– 2056, 2015
work page 2015
-
[1]
Communication and control co-design for networked control systems,
L. Zhang and D. Hristu-Varsakelis, “Communication and control co-design for networked control systems,”Automatica, vol. 42, no. 6, pp. 953–958, 2006. 16
work page 2006
-
[2]
Event-triggered communication andh ∞ control co-design for net- worked control systems,
C. Peng and T. C. Yang, “Event-triggered communication andh ∞ control co-design for net- worked control systems,”Automatica, vol. 49, no. 5, pp. 1326–1332, 2013
work page 2013
-
[3]
Communication delay co-design inH 2-distributed control using atomic norm minimization,
N. Matni, “Communication delay co-design inH 2-distributed control using atomic norm minimization,”IEEE Transactions on Control of Network Systems, vol. 4, no. 2, pp. 267–278, 2015
work page 2015
-
[4]
N. Matni and V. Chandrasekaran, “Regularization for design,”IEEE Trans. Autom. Control, vol. 61, no. 12, pp. 3991–4006, 2016
work page 2016
-
[5]
Jointly optimal LQG quantization and control policies for multi-dimensional sys- tems,
S. Yüksel, “Jointly optimal LQG quantization and control policies for multi-dimensional sys- tems,”IEEE Trans. Autom. Control, vol. 59, pp. 1612–1617, 2013
work page 2013
-
[6]
Lack of separation principle for quantized linear quadratic Gaussian control,
M. Fu, “Lack of separation principle for quantized linear quadratic Gaussian control,”IEEE Transactions on Automatic Control, vol. 57, no. 9, pp. 2385–2390, 2012
work page 2012
-
[7]
Optimal controller synthesis and dynamic quantizer switching for linear-quadratic-gaussian systems,
D. Maity and P . Tsiotras, “Optimal controller synthesis and dynamic quantizer switching for linear-quadratic-gaussian systems,”IEEE Transactions on Automatic Control, vol. 67, no. 1, pp. 382–389, 2021
work page 2021
Show all 34 references
-
[8]
Learning to communicate with deep multi-agent reinforcement learning,
J. Foerster, I. A. Assael, N. De Freitas, and S. Whiteson, “Learning to communicate with deep multi-agent reinforcement learning,” inNeurIPS, 2016
2016
-
[9]
Learning multiagent communication with back- propagation,
S. Sukhbaatar, A. Szlam, and R. Fergus, “Learning multiagent communication with back- propagation,” inNeurIPS, 2016
2016
-
[10]
Decentralized stochastic control with partial history sharing: A common information approach,
A. Nayyar, A. Mahajan, and D. Teneketzis, “Decentralized stochastic control with partial history sharing: A common information approach,”IEEE Trans. Autom. Control, vol. 58, no. 7, pp. 1644–1658, 2013
2013
-
[11]
Common information based Markov perfect equilibria for linear-gaussian games with asymmetric information,
A. Gupta, A. Nayyar, C. Langbort, and T. Basar, “Common information based Markov perfect equilibria for linear-gaussian games with asymmetric information,”SIAM Journal on Control and Optimization, vol. 52, no. 5, pp. 3228–3260, 2014
2014
-
[12]
Optimal communication and control strate- gies in a cooperative multiagent MDP problem,
S. Sudhakara, D. Kartik, R. Jain, and A. Nayyar, “Optimal communication and control strate- gies in a cooperative multiagent MDP problem,”IEEE Transactions on Automatic Control, vol. 69, no. 10, pp. 6959–6966, 2024
2024
-
[13]
Optimal communication and control strate- gies for a multi-agent system in the presence of an adversary,
D. Kartik, S. Sudhakara, R. Jain, and A. Nayyar, “Optimal communication and control strate- gies for a multi-agent system in the presence of an adversary,” inIEEE Conf. on Dec. and Control, 2022
2022
-
[14]
Principled learning-to-communicate with quasi-classical in- formation structures,
X. Liu, H. You, and K. Zhang, “Principled learning-to-communicate with quasi-classical in- formation structures,” in2025 64th IEEE Conference on Decision and Control (CDC). IEEE, 2025
2025
-
[15]
Team decision theory and information structures in optimal control problems – part I,
Y.-C. Ho and K.-C. Chu, “Team decision theory and information structures in optimal control problems – part I,”IEEE Trans. Autom. Control, vol. 17, pp. 15–22, 1972
1972
-
[16]
Optimal decentralized state-feedback control with sparsity and delays,
A. Lamperski and L. Lessard, “Optimal decentralized state-feedback control with sparsity and delays,”Automatica, pp. 143–151, 2015. 17
2015
-
[18]
Structural results for partially nested LQG systems over graphs,
A. Nayyar and L. Lessard, “Structural results for partially nested LQG systems over graphs,” in2015 American Control Conference (ACC). IEEE, 2015, pp. 5457–5464
2015
-
[19]
Partially observable multiagent reinforcement learning with informa- tion sharing,
X. Liu and K. Zhang, “Partially observable multiagent reinforcement learning with informa- tion sharing,”SIAM Journal on Control and Optimization, vol. 64, no. 2, pp. 673–697, 2026
2026
-
[20]
Yüksel and T
S. Yüksel and T. Başar,Stochastic Teams, Games, and Control under Information Constraints. Springer Nature, 2023
2023
-
[21]
On information structures, feedback and causality,
H. S. Witsenhausen, “On information structures, feedback and causality,”SIAM Journal on Control, vol. 9, no. 2, pp. 149–160, 1971
1971
-
[22]
Information structures in opti- mal decentralized control,
A. Mahajan, N. C. Martins, M. C. Rotkowitz, and S. Yüksel, “Information structures in opti- mal decentralized control,” inIEEE Conf. on Dec. and Control, 2012
2012
-
[23]
A counterexample in stochastic optimum control,
H. S. Witsenhausen, “A counterexample in stochastic optimum control,”SIAM Journal on Control, vol. 6, no. 1, pp. 131–147, 1968
1968
-
[24]
Common information based Markov perfect equilibria for stochastic games with asymmetric information: Finite games,
A. Nayyar, A. Gupta, C. Langbort, and T. Başar, “Common information based Markov perfect equilibria for stochastic games with asymmetric information: Finite games,”IEEE Trans. Autom. Control, vol. 59, pp. 555–570, 2013
2013
-
[25]
Axler,Linear Algebra Done Right, 4th ed
S. Axler,Linear Algebra Done Right, 4th ed. Springer, 2024
2024
-
[26]
The approximation of one matrix by another of lower rank,
C. Eckart and G. Young, “The approximation of one matrix by another of lower rank,”Psy- chometrika, vol. 1, no. 3, pp. 211–218, 1936
1936
-
[27]
On Kalman filtering for conditionally gaussian systems with random matrices,
H.-F. Chen, P . R. Kumar, and J. H. van Schuppen, “On Kalman filtering for conditionally gaussian systems with random matrices,”Systems & Control Letters, vol. 13, no. 5, pp. 397– 404, 1989
1989
-
[28]
Structure and stability of discrete-time optimal sys- tems,
D. Rappaport and L. M. Silverman, “Structure and stability of discrete-time optimal sys- tems,”IEEE Transactions on Automatic Control, vol. 16, no. 3, pp. 227–233, Jun. 1971
1971
-
[29]
The generalised discrete algebraic riccati equation aris- ing in lq optimal control problems: Part i,
A. Ferrante and L. Ntogramatzidis, “The generalised discrete algebraic riccati equation aris- ing in lq optimal control problems: Part i,” in2012 IEEE 51st IEEE Conference on Decision and Control (CDC). IEEE, 2012, pp. 6394–6399. 18 A. Decentralized Linear-Quadratic-Gaussian C...
2012
-
[30]
•Dsatisfies Assumption III.2:The only zero input coefficient relevant to a later decision is the scalarB 2,1 = 0, andU 2,1 does not appear in any later information set
Then, we can verify that: •Dsatisfies Assumption II.1. •Dsatisfies Assumption III.2:The only zero input coefficient relevant to a later decision is the scalarB 2,1 = 0, andU 2,1 does not appear in any later information set. •Dsatisfies Assumption III.4:It holds that rank(E 1,2...
-
[31]
Therefore, we know that underg m,∗ 1:2 , we haveI i,1+ ={Y i,1},∀i∈[2],I 2,2+ ={Y 2,1,U 2,1,Y 2,2,U 1,1}
Secondly, we can assume gm,∗ 1,2 = (0,1,0), otherwise we can change it to be (0,1,0) and it is still a team-optimal strategy, since additionally sharingU 1,1 enlarges theI 2,2+ but incurs no communication cost. Therefore, we know that underg m,∗ 1:2 , we haveI i,1+ ={Y i,1},∀i...
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[32]
•Dhas PN IS:If there is no additional sharing, then for anyi 1,i 2∈[2],I i1,1−⊆I i2,2− ifi 1 =i 2; otherwise, agent (i1,1) does not influence agent (i2,2)
Then, we can verify that: •Dsatisfies Assumption II.1. •Dhas PN IS:If there is no additional sharing, then for anyi 1,i 2∈[2],I i1,1−⊆I i2,2− ifi 1 =i 2; otherwise, agent (i1,1) does not influence agent (i2,2). •Dsatisfies Assumption III.2:The only zero input coefficient relev...
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[33]
Suppose the optimal communication strategyg m,∗ 1:2 yields that agents shareY 1,2 through additional sharing, i.e.Z a 1 =∅,Z a 2 ={Y 1,2}. Then, finding a team-optimal strategy ofDcan be reduced to finding an optimal strategy of the Decentralized LQG problem in Proposition B.2...
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[34]
eAh eBh eF∗ h eL3 h+1eEh+1eAh eL1 h+1 + eL2 h+1 eF∗ h + eL3 h+1eEh+1eBh eF∗ h # , ˘Bh =
Suppose the optimal communication strategyg m,∗ 1:2 yields that agents share nothing through additional sharing, i.e.,Z a 1 =Z a 2 =∅. Then, letg a,∗ 1:2 be the optimal control strategy, and letg m,′ 1:2 be the communication strategy that additionally sharesY 1,2. Since bothg ...
Reviewed August 14, 2026 · model on record in the stance chip above.
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