REVIEW 2 major objections 5 minor 17 references
Robust Deterministic Policies for Markov Decision Processes under Budgeted Uncertainty
T0 review · 2 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper claims that finding an optimal deterministic policy in the LDST budgeted-uncertainty MDP model is computationally intractable, with NP-hardness already for two stages and a single deviating terminal reward, and…
desk verdict Hardness results are solid, but the approximation theorem has an unstated nonnegativity assumption that currently makes it false as written. 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 argument for hardness is carried by reductions from classic problems: 3-Partition for the two-stage NP-hardness, Vertex-Disjoint Paths for the inapproximability of the general reward case, Max-Min Vertex Cover for the Sigma_2^p-hardness of transition uncertainty, and 3-SAT for the fixed-budget decision version. The positive algorithm is organized around the loss of a policy, $L(\pi) = R(\pi) - \hat{R}(\pi)$, the drop in expected reward under the worst-case deviation. It solves a Knapsack Cover relaxation (with an FPTAS) for instances where the optimal loss is large relative to the optimal worst-case reward, and a Generalized Assignment relaxation (with a standard rounding for assignment problems) for instances where the loss is small; the better of the two resulting policies is returned.
What would settle it
For a terminal state $t$ with nominal reward $r(t)=1$, deviating reward $r'(t)=-1$, and a policy reaching $t$ with probability 1, the paper's reduction yields $h=0$ and replaces $t$ by a state with nominal reward 2 and deviating reward 0, changing the worst-case value from $-1$ to $0$; any 2-Stage Reward-DLP instance containing such a negative deviation therefore tests whether Theorem 3's guarantee actually covers the paper's stated domain of arbitrary real rewards, and the described reduction shows it does not.
Extended reading notes
Core claim
The central discovery is a separation between randomized and deterministic policies in the LDST model. Whereas optimal randomized policies under reward uncertainty can be computed efficiently, the deterministic version is intractable: the two-stage reward case is strongly NP-hard (Theorem 1), the general reward case admits no constant-factor approximation unless P = NP (Theorem 2), and the version with uncertain transitions is Sigma_2^p-hard (Theorem 5), with a further NP-hardness result for deciding positive robust reward with budget k=2 (Theorem 4). The only general positive result is a 1/(5+epsilon)-approximation for the two-stage reward case (Theorem 3), obtained by running two complementary relaxations and taking the better policy.
Load-bearing premise
The paper's positive constant-factor approximation for two-stage reward uncertainty depends on an unstated assumption that every deviating reward is nonnegative and no larger than the nominal reward, since its reduction to zero deviations splits a terminal state into an integer number of copies using $h = \lceil r'(t)/(r(t)-r'(t)) \rceil$, a formula that is not value-preserving when $r'(t) < 0$.
Editorial extensions
If this is right
- Even the simplest meaningful deviation—a single terminal reward changing in a two-stage MDP—makes the optimal deterministic policy impossible to compute in polynomial time unless P = NP.
- For general reward uncertainty, no polynomial-time algorithm can guarantee any fixed fraction of the optimal worst-case reward, so approximation must be restricted to special structures.
- For transition uncertainty, the Sigma_2^p-hardness implies that no compact mixed-integer programming formulation can capture the problem unless the polynomial hierarchy collapses.
- The 1/(5+epsilon)-approximation for the two-stage reward case shows that this restricted problem is approximable despite being NP-hard, and the approximation factor is independent of the size of the MDP.
Reading between the lines
- If the unstated nonnegativity of deviating rewards is genuinely required, the positive approximation result is less general than the problem definition suggests; extending it to negative deviations would need a different rounding or a different relaxation.
- The hardness contrast suggests that randomization in robust MDPs is not merely a modelling convenience but a source of tractability, which may matter for applications that require deterministic execution.
- The two-relaxation idea—one for large-loss and one for small-loss regimes—could plausibly be adapted to other budgeted-robust optimization problems where a similar loss measure is available.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the computational complexity of finding optimal deterministic policies in the Lightning Does Not Strike Twice (LDST) robust MDP model under budgeted uncertainty. It proves that 2-Stage Reward-DLP is strongly NP-hard (Theorem 1), that Reward-DLP has no polynomial-time constant-factor approximation unless P=NP (Theorem 2), that Transition-DLP is NP-hard even for budget k=2 (Theorem 4), and that Transition-DLP is Sigma_2^p-hard (Theorem 5). On the positive side, it gives a 1/(5+epsilon)-approximation algorithm for 2-Stage Reward-DLP (Theorem 3), based on a Knapsack Cover relaxation and a Generalized Assignment relaxation. The hardness reductions appear sound and are mostly clean; the main issue is that the positive result relies on an unstated nonnegativity assumption that is not part of the model as defined.
Significance. If the results stand, they are significant: they sharply separate deterministic from randomized policies in the LDST model, complementing Mannor et al.'s polynomial-time result for randomized policies. The Sigma_2^p-hardness of Transition-DLP is a strong statement about the impossibility of compact mixed-integer formulations. The approximation algorithm for the two-stage reward case is a useful positive counterpoint to the hardness results. The paper's reductions are from established hard problems and use external algorithms (FPTAS for Knapsack Cover and the Shmoys-Tardos algorithm for Generalized Assignment), which makes the proofs verifiable. However, the positive result currently depends on an assumption that is absent from the model definition, and one central reduction contains a misstated equation; these issues are fixable but require revision.
major comments (2)
- [Section 5.1, Assumption 2; Theorems 3, 7, 8] The reduction to r'(t)=0 is not without loss of generality for the model defined in Sections 2.1-2.3. The model allows arbitrary real terminal rewards and only requires r'(s) <= r(s). The transformation in Section 5.1 is applied only for r'(t)>0; for r'(t)<0, the formula h = ceil(r'(t)/(r(t)-r'(t))) is non-positive or undefined, and the replacement states and probabilities are not well-defined. This is not a cosmetic gap: in the two-terminal instance with S1={s}, S2={t1,t2}, p(t1)=p(t2)=1/2, r(t1)=r(t2)=1, r'(t1)=0, r'(t2)=-1, and k=1, the true robust value is 0, L(pi)=1, but no policy can achieve nominal reward at least 1 on a single terminal, so Lemma 10 fails. The enumeration in Theorems 7 and 8 also assumes r_min>0 in the definition of L_i, which is not guaranteed by the model. Theorem 3 therefore needs an explicit assumption such as r'(t) >= 0 and r(t) > r'(t) (which implies nonnegative nominal rewards), or the transformation and guarantees must be extended to negative alternative rewards.
- [Section 6, proof of Theorem 5] The sentence identifying the choice of epsilon is incorrect as written. The text states that epsilon is 'the unique solution to 1 - |I|*epsilon/|V| = epsilon/(2m)', but the value epsilon = 2m/(2m|I|+|V|) does not satisfy this equation in general; for example, with m=1, |I|=1, |V|=10, the left-hand side is approximately 0.983 while the right-hand side is approximately 0.083. The inequalities later needed (p_e > p_v for e in E_{t_pi} and p_e < p_v otherwise) do hold for the stated epsilon because p_v = 1/(2m|I|+|V|), p_e = 2epsilon/(3m) = 4/(3(2m|I|+|V|)) for e in E_{t_pi}, and p_e <= epsilon/(3m) = 2/(3(2m|I|+|V|)) otherwise. The proof is thus repairable, but the present derivation does not justify the claim and should be corrected.
minor comments (5)
- [Section 4, proof of Theorem 1] In the definition of the state space, 'S3 := {tj}' should be 'S2 := {tj}', since the three stages are S0, S1, and S2.
- [Section 5.3, definition of UB2] The artificial state t* is denoted with the same star symbol used in v_{st}^*, which makes the notation confusing in the definition of v_{st}^* and in Lemmas 12 and 13; please use distinct notation for the artificial state.
- [Section 5.3, proof of Theorem 8] In the displayed inequality after invoking the Shmoys-Tardos algorithm, the right-hand side should be the objective value of the fractional solution, not the same expression with y' on both sides. Also, the enumeration in the proof of Theorem 8 loops over every t-hat in S2, but UB2(L) does not depend on t-hat; only the L_i values are needed.
- [Section 6, proof of Theorem 4] In the definition of A', 'for some i in N' should be 'for some i in M' (the clauses), and the notation 'pi(sCi)' should be 'pi(s_{c_i})' for consistency.
- [Throughout] There are several typographical slips: 'there a is a 1/(5+epsilon)-approximation' in Theorem 3, and the expression 'pv = 1 - epsilon|I| / |V|' in the proof of Theorem 5 should be parenthesized as '(1 - epsilon|I|)/|V|'.
Circularity Check
No significant circularity: all hardness reductions and approximation guarantees rest on external problems and external algorithms.
full rationale
The paper's derivation chain is self-contained against external benchmarks rather than against its own inputs. Theorem 1 is proved by a direct reduction from 3-Partition, Theorem 2 from Vertex-Disjoint Paths, Theorem 4 from 3-SAT, and Theorem 5 from Max-Min Vertex Cover; each of these is an established NP-hard or Sigma_2^p-complete problem, and the reductions construct LDST instances whose robust-policy values correspond to the source instances' answers. The approximation result for 2-Stage Reward-DLP does not fit any parameter to the quantity it later calls a prediction. Theorems 7 and 8 derive worst-case guarantees from two relaxations, UB1 and UB2, and the algorithm combines them via Lemma 9; the relaxed subproblems are solved with the external FPTAS for Knapsack Cover and the external Shmoys-Tardos algorithm for Generalized Assignment, respectively. The bounds are proven directly: Lemma 10 shows UB1 gives an upper bound on the robust reward for suitable guessed loss values, Lemma 11 converts a UB1 solution back to a policy with guaranteed robust reward, Lemma 12 shows UB2 upper-bounds half the nominal reward, and Lemma 13 converts a UB2 solution to a policy losing at most the guessed loss. None of these steps equates the claimed result to a fitted or self-referential quantity by construction. The paper's use of Mannor et al. is contextual (defining LDST and contrasting the randomized-policy setting) and is not load-bearing for any new theorem. The reviewer-identified weakness concerning the unstated nonnegativity condition in the 'without loss of generality' reduction to r'(t)=0 is a genuine assumption gap and a potential correctness issue, but it is not circularity: the approximation proof would be invalid on a larger instance class, not circular because it assumes what it proves. Therefore the circularity score is 0.
Assumptions & free parameters
assumptions (8)
- standard math FPTAS for Knapsack Cover (Pruhs and Woeginger 2007)
- standard math Shmoys-Tardos approximation algorithm for Generalized Assignment Problem
- standard math NP-hardness of 3-Partition, Vertex-Disjoint Paths, and 3-SAT, and Sigma_2^p-completeness of Max-Min Vertex Cover
- standard math Woeginger's theorem: Sigma_2^p-hard optimization problems admit no compact MIP formulation unless the polynomial hierarchy collapses
- domain assumption Nonnegativity of rewards (implicit)
- domain assumption LDST budgeted uncertainty model
- domain assumption Finite-horizon, layered (acyclic) MDP structure
- domain assumption Deterministic, history-independent policies as the policy class
Cite this review
Pith. "Pith review of Robust Deterministic Policies for Markov Decision Processes under Budgeted Uncertainty." pith.science (2026). https://pith.science/paper/XQ3P4R2X
@misc{pith2026241212879,
author = {Pith},
title = {Pith review of: Robust Deterministic Policies for Markov Decision Processes under Budgeted Uncertainty},
year = {2026},
howpublished = {\url{https://pith.science/paper/XQ3P4R2X}},
note = {Machine review of arXiv:2412.12879}
}
abstract
This paper studies the computation of robust deterministic policies for Markov Decision Processes (MDPs) in the Lightning Does Not Strike Twice (LDST) model of Mannor, Mebel and Xu (ICML '12). In this model, designed to provide robustness in the face of uncertain input data while not being overly conservative, transition probabilities and rewards are uncertain and the uncertainty set is constrained by a budget that limits the number of states whose parameters can deviate from their nominal values. Mannor et al. (ICML '12) showed that optimal randomized policies for MDPs in the LDST regime can be efficiently computed when only the rewards are affected by uncertainty. In contrast to these findings, we observe that the computation of optimal deterministic policies is $N\!P$-hard even when only a single terminal reward may deviate from its nominal value and the MDP consists of $2$ time periods. For this hard special case, we then derive a constant-factor approximation algorithm by combining two relaxations based on the Knapsack Cover and Generalized Assignment problem, respectively. For the general problem with possibly a large number of deviations and a longer time horizon, we derive strong inapproximability results for computing robust deterministic policies as well as $\Sigma_2^p$-hardness, indicating that the general problem does not even admit a compact mixed integer programming formulation.
Figures
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Reference graph
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Reviewed August 11, 2026 · model on record in the stance chip above.
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