REVIEW 4 major objections 5 minor 17 references
Lower Bounds against the Ideal Proof System in Finite Fields
T0 review · 4 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read This paper proves that no polynomial-size constant-depth multilinear Ideal Proof System refutation of a knapsack-mod-p polynomial exists over any fixed finite field of characteristic at least 5.
desk verdict Real progress on IPS over finite fields, but the main lower bound currently rests on a false corollary and an unproved balanced-word construction; salvageable, not there yet. 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 knapsack-mod-p polynomial $\mathrm{ks}_{w,p}$, together with the relative rank of the coefficient matrix of its reciprocal—that is, the rank divided by the square root of the product of the row and column counts. The word $w$ must be balanced, meaning every index has at least one overlap with an index of the opposite sign, and must admit a scattered partition of its positive indices into fewer than p parts; these conditions make the embedded p-1 powers Boolean functions and let the instance be shifted to an unsatisfiable one. The argument's engine is a rank lower bound (Lemma 21): for the multilinear polynomial $f$ that agrees with $1/\mathrm{ks}_{w,p}$ on Boolean assignments, the matrix $M_w(f)$ has full rank, by an induction on submonomials of the negative variables. Full rank feeds through the set-multilinearization result [For24] and the improved set-multilinear-to-rank bound [BDS24], and the final size lower bound follows; the carrying identity is the full-degree lemma, which says that a multilinear polynomial inverse of a sum of full-degree Boolean functions minus a constant must itself have full degree.
What would settle it
Exhibit a balanced word $w$ over $\{\alpha k,-k\}$ in the parameter range of Theorem 24 whose positive positions cannot be partitioned into fewer than $p$ groups with pairwise disjoint overlaps; that single example would break the construction of $\mathrm{ks}_{w,p}$ and the rank lower bound. Alternatively, for a fixed such word, compute the minimal size of a product-depth-$\Delta$ set-multilinear circuit computing the projection $\Pi_w(1/\mathrm{ks}_{w,p})$: a polynomial-size circuit in $n$ would disprove Lemma 26.
Extended reading notes
Core claim
The paper's central claim is Theorem 24: for every prime p>=5 and every field F of characteristic p, any product-depth at most $\Delta$ multilinear $\mathrm{IPS}_{\mathrm{LIN}}'$ refutation over F of the knapsack-mod-p instance $\mathrm{ks}_{w,p}$ has size at least $n^{\Omega(\lambda/\Delta)}$, where $\Delta \le \log\log\log n / 4$ and $\lambda = \lfloor d^{1/G(\Delta)} \rfloor$ with $d = \lfloor \log n / 4 \rfloor$. The hard instance $\mathrm{ks}_{w,p}$ is built from a balanced integer word $w$ over the two-symbol alphabet $\{\alpha k, -k\}$, with a scattered partition of its positive indices into fewer than p parts, and with each summand raised to the p-1 power so that Fermat's little theorem forces it to be Boolean-valued; a suitable shift $\beta$ then makes the whole polynomial unsatisfiable over Boolean assignments. The lower-bound argument reduces refutation size to a full-rank statement about the coefficient matrix of the reciprocal function $1/\mathrm{ks}_{w,p}$ over Boolean assignments, and then rules out small set-multilinear circuits for that projection using a set-multilinearization theorem that works over all fields [For24] together with improved rank-to-size parameters [BDS24]. The paper also proves a separation (Theorem 36): the degree-2 symmetric knapsack $\mathrm{ks}_{w,e2}$ has polynomial-size constant-depth multilinear $\mathrm{IPS}_{\mathrm{LIN}}'$ refutations over any field of characteristic p>=3, but requires super-polynomial size over every characteristic-zero field.
Load-bearing premise
The weakest load-bearing premise is a one-sentence assertion in the proof of Theorem 24 that a balanced integer word $w$ over $\{\alpha k,-k\}$ with exactly $d$ positions and with its positive positions partitionable into fewer than $p$ disjoint groups exists; the construction is not given, and if no such word exists for the stated parameters the rank lower bound, and with it the finite-field lower bound, would not go through.
Editorial extensions
If this is right
- For every fixed prime p>=5, the knapsack-mod-p polynomial is a concrete super-polynomial hard instance for constant-depth multilinear $\mathrm{IPS}_{\mathrm{LIN}}'$ over all fields of characteristic p.
- The same instance also stays hard over characteristic-zero fields, so it supplies new hard instances for the earlier characteristic-zero proof system as well.
- The degree-2 symmetric knapsack separates the two settings: it has short constant-depth multilinear refutations over fields of characteristic at least 3, but all such refutations over characteristic-zero fields require super-polynomial size.
- Over any fixed finite field, explicit instances force any roABP-$\mathrm{IPS}_{\mathrm{LIN}}'$ refutation, in any variable order, to have exponential size; the paper also shows the functional lower bound method alone cannot produce non-placeholder versions of these bounds over finite fields.
- If any instance is shown hard for non-multilinear bounded-depth IPS over a finite field, the translation lemma converts it into a hard CNF and hence, by known simulations, into an AC^0[p]-Frege lower bound.
Reading between the lines
- Beyond the paper: the unproved balanced-word existence claim, if it fails for some parameters, would not necessarily destroy the whole lower-bound method; a different word family or a different scattered partition might restore the rank argument, and testing the construction explicitly for small d and k is a cheap way to check the route.
- Beyond the paper: the translation lemma's removal of extension axioms suggests that any proof system that can internally derive the finite-field axioms and Lagrange interpolation identities can convert algebraic-instance lower bounds into propositional ones; it would be natural to see whether the same internal bit-arithmetic works for polynomial calculus or Nullstellensatz fragments over finite fi
- Beyond the paper: the separation instance $\mathrm{ks}_{w,e2}$ shows that the characteristic of the field can change the complexity of the same algebraic instance; one could look for a family whose required refutation size varies with p, giving a finer map of how IPS strength depends on the ground field.
- Beyond the paper: the paper's limitation statement for roABP-IPS means that non-placeholder roABP-IPS lower bounds over finite fields would need a method that is not the functional lower bound method; lower-bound-by-multiples is the natural candidate, but it has not yet been pushed to non-placeholder instances in this setting.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies lower bounds for fragments of the Ideal Proof System over finite fields. The central results claimed are: (1) a super-polynomial lower bound for constant-depth multilinear IPS_LIN' refutations of a knapsack mod p instance over any field of fixed characteristic p≥5 (Theorem 24); (2) a separation of this system over finite fields from the characteristic-zero system via a symmetric knapsack instance (Theorem 36); (3) exponential lower bounds for roABP-IPS_LIN' over finite fields, in both fixed and arbitrary variable orders (Theorems 43 and 46), together with a limitation of the functional method (Theorem 49); and (4) a translation lemma showing that algebraic-instance lower bounds against bounded-depth IPS over finite fields imply CNF lower bounds and hence AC^0[p]-Frege lower bounds (Theorem 60). The proofs combine the functional lower bound method, Forbes's set-multilinearization over all fields, the BDS24 rank parameters, and the GHT22 framework.
Significance. If the missing pieces are supplied, these are substantial results: they would give the first lower bounds for constant-depth multilinear IPS over any fixed finite field, removing the large-characteristic assumption that underpins earlier work, and the roABP results give simple finite-field analogues of previous large-field bounds. The translation lemma is a clean conceptual contribution that removes the extension axioms from the earlier ST25 translation. The paper proves several key technical lemmas in detail (Lemma 19, Lemma 21, Lemma 33) and correctly exploits Forbes's set-multilinearization result and the BDS24 improved parameters. However, two load-bearing proofs are only sketched or omitted (the balanced-word construction inside Theorem 24, and Lemma 35), and there is an exponent mismatch in Lemma 26, so the manuscript is not yet in a publishable state.
major comments (4)
- [§3.5, proof of Theorem 24] The proof contains the sentence 'Construct, by induction, a balanced word w in Z^d over the alphabet {αk,-k}' but gives neither the construction nor a reference to a lemma establishing it. This is load-bearing: Lemma 26 needs a balanced word with those block lengths, Corollary 23 needs the resulting balance to imply |w_P|-|w_N| ≥ -b, and the scattered partition in the comment after Theorem 24 needs Δ_G(N_w)≤3. Please provide an explicit inductive construction with these parameters, or identify the exact lemma in [GHT22]/[BDS24] and verify that its parameters match d=⌊log n/4⌋ and the stated ranges of α and k.
- [§4.2, Lemma 35] The proof of Lemma 35 is omitted ('essentially the same as the proof of Theorem 24 ... and is omitted here'). Since the degree lower bound Lemma 33 is structurally different from the knapsack-mod-p degree argument, and since Theorem 36 is the paper's separation claim, the reduction needs to be written out. At minimum, specify which parts of the Theorem 24 proof carry over verbatim and where Lemma 33 is invoked.
- [§3.5, Lemma 26] The inequality chain in the proof gives 2^{-k} ≤ rel-rank(F) ≤ s 2^Δ 2^{-kλ/256}, which yields s ≥ 2^{k(λ/256-1)-Δ}, not the stated s ≥ 2^{k(λ/256-1)/(2Δ)}. The discrepancy should be corrected or explained; as written, the statement of Lemma 26 does not follow from its proof.
- [§3.4, Corollary 23] The implication 'w is balanced and |w_i|≤b implies |w_P|-|w_N| ≥ -b' is asserted without proof. It is true under the contiguous-interval interpretation of A(i)_w and B(j)_w (the negative intervals partition [1,|w_N|], so |w_N|>|w_P|+b would leave the last negative interval disjoint from all positive intervals), but the argument should be included, since this inequality is what converts full rank into the relative-rank bound rel-rank ≥ 2^{-b/2}.
minor comments (5)
- [Throughout §3 and §4] Several lemmas are cited as 'Theorem 18', 'Theorem 19', 'Theorem 20', and 'Theorem 33'; these cross-references should be changed to 'Lemma'.
- [§5.4] The word 'acheive' should be 'achieve'.
- [§5.2, Corollary 43] The phrase 'constant characteristics q' should be 'constant characteristic q'.
- [§6, Theorem 59] The notation SCNF(C(x)) is used in the proof where Definition 58 writes SCNF(C(x)=0); please make the notation uniform.
- [§3.5, Lemma 28] The quoted set-multilinearization bound poly(s, Θ(d/ln d)^d) would benefit from an explicit statement of the dependence of implicit constants on the field, because Theorem 24 later converts this bound into a concrete n^{Ω(λ/Δ)} lower bound.
Circularity Check
No significant circularity: the finite-field knapsack lower bound rests on new instance analysis and external published lemmas; the load-bearing balanced-word construction is a correctness gap, not a definitional reduction.
full rationale
I walked the main derivation chain: Theorem 24 reduces to Theorem 25, which reduces to Lemma 26, which uses Lemma 21/Corollary 23 for a full-rank/relative-rank lower bound on M_w(f), and Lemma 19/20 for the degree lower bound. These rank and degree statements are proved in the paper, not assumed as the target result. The external inputs, Forbes' set-multilinearization over all fields (Lemma 28) and the BDS24/LST21 relative-rank bound (Claim 27), are published results with independent derivations, so citing them is genuine support under the hard rules. The hard instance ks_{w,p} differs from the GHT22 knapsack by the Fermat power (p-1), and the paper explicitly addresses the finite-field obstacle that the knapsack is Boolean-satisfiable; this is a real modification rather than a renaming. The self-citations to GHT22 and ST25 are present, but the finite-field proofs are re-derived internally and the translation lemma in Section 6 eliminates extension axioms with new Lagrange-polynomial arguments, so the central claims do not reduce by construction to those papers. Separately, I flag a non-circular correctness gap: the proof of Theorem 24 contains only 'Construct, by induction, a balanced word w in Z^d over the alphabet {alpha k, -k}', without giving the induction, and Corollary 23's assertion that balance plus |w_i| <= b implies |w_P|-|w_N| >= -b is unsupported and appears false for balanced alternating words with alpha = 1/2 and large d. This is an omitted proof / unsupported inequality affecting whether Lemma 26's hypotheses are met; it is a correctness risk, not a circular derivation, so the circularity score remains 1.
Assumptions & free parameters
free parameters (4)
- beta (knapsack mod p) =
an element of F not in {0,...,r}
- beta (symmetric knapsack) =
an element of F such that e_2(x)-beta is unsatisfiable
- constant 2 in roABP hard instance f = prod(1-x_i) - 2 =
2
- alpha and k (word parameters) =
alpha in [1/2,1), k in [floor(log n)/2, floor(log n)], alpha*k integer
assumptions (9)
- standard math Hilbert Nullstellensatz over finite fields
- standard math Fermat's little theorem
- standard math Lucas's theorem
- domain assumption Forbes set-multilinearization over any field (Lemma 28, [For24])
- domain assumption BDS24/LST21 relative-rank bound (Claim 27, [BDS24] Lemma 4.3)
- domain assumption FSTW21 full-degree bound for subset-sum (Lemma 18, [FSTW21] Proposition 5.3)
- domain assumption FSTW21 functional lower bound method (Theorem 10)
- domain assumption FSTW21 read-twice oblivious ABP lower bound (Lemma 44)
- domain assumption ST25 translation lemma for extended CNFs (Lemma 56)
invented entities (4)
-
ks_{w,p} (knapsack mod p)
-
ks_{w,e2} (symmetric knapsack of degree 2)
-
f = prod(1-x_i) - 2 and its lifted version f^*(z,x)
-
unsatisfiable system (f,g,x^2-x) with f=prod_{i<j}(x_i+x_j+1), g=prod(1-x_i)-1
Cite this review
Pith. "Pith review of Lower Bounds against the Ideal Proof System in Finite Fields." pith.science (2026). https://pith.science/paper/HFD3TTJK
@misc{pith2026250617210,
author = {Pith},
title = {Pith review of: Lower Bounds against the Ideal Proof System in Finite Fields},
year = {2026},
howpublished = {\url{https://pith.science/paper/HFD3TTJK}},
note = {Machine review of arXiv:2506.17210}
}
abstract
Lower bounds against strong algebraic proof systems and specifically fragments of the Ideal Proof System (IPS), have been obtained in an ongoing line of work. All of these bounds, however, are proved only over large (or characteristic $0$) fields, yet finite fields are the more natural setting for propositional proof complexity, especially for progress toward lower bounds for Frege systems such as $AC^0[p]$-Frege. This work establishes lower bounds against fragments of IPS over fixed finite fields. Specifically, we show that a variant of the knapsack instance studied by Govindasamy, Hakoniemi, and Tzameret (FOCS'22) has no polynomial-size IPS refutation over finite fields when the refutation is multilinear and written as a constant-depth circuit. The key ingredient of our argument is the recent set-multilinearization result of Forbes (CCC'24), which extends the earlier result of Limaye, Srinivasan, and Tavenas (FOCS'21) to all fields, and an extension of the techniques of Govindasamy, Hakoniemi, and Tzameret to finite fields. We also separate this proof system from the one studied by Govindasamy, Hakoniemi, and Tzameret. In addition, we present new lower bounds for read-once algebraic branching program refutations, roABP-IPS, in finite fields, extending results of Forbes, Shpilka, Tzameret, and Wigderson (Theor. of Comput.'21) and Hakoniemi, Limaye, and Tzameret (STOC'24). Finally, we show that any lower bound against any proof system at least as strong as (non-multilinear) constant-depth IPS over finite fields for any instance, even a purely algebraic instance (i.e., not a translation of a Boolean formula or CNF), implies a hard CNF formula for the respective IPS fragment, and hence an $AC^0[p]$-Frege lower bound by known simulations over finite fields (Grochow and Pitassi (J. ACM'18)).
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