REVIEW 2 major objections 4 minor 26 references
On Optimal Testing of Linearity
T0 review · 2 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read This paper gives optimal linearity testers that tolerate online manipulation for almost the full range of budgets, and cuts real-domain linearity testing to O(1/ε) queries.
desk verdict The online-manipulation half is solid and new; the real-valued additivity theorem has a completeness bug that needs fixing before it is true as stated. 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 machinery is a pair of regime-shifting primitives plus a self-correction object. $\mathrm{XorTest}_k$, the $k$-point test, chooses $k$ random points and checks the parity constraint $f(\oplus_i x_i)=\prod_i f(x_i)$; its rejection probability is amplified by a factor $\approx k$ over the 3-point test, which is what makes $O(\log t + 1/\varepsilon)$ queries attainable when $t$ is small. Sample-based testing, in the form of the Goldreich--Ron tester, draws $\Theta(n+1/\varepsilon)$ uniformly random labeled samples and either extrapolates a linear function from a basis or rejects; because all queries are random and independent of previous answers, the probability that any sample lands on an adversarially manipulated coordinate is at most $q^2 t/2^n$, so the adversary is neutralized for large $t$. Over the reals, the central object is the self-corrected function $g(p)=\mathrm{maj}_{x\sim N(0,I)} \kappa_p (f(p/\kappa_p - x)+f(x))$, where $\kappa_p$ radially contracts $p$ into a small ball; a single random direction $x$ gives the 'opinion' $g_x(p)$, and the paper shows one such opinion is enough per test point. The impossibility direction uses Fischer's hard-distribution argument, which concentrates on the $\ell$ least-queried disagreement points and lets the adversary hide them within the manipulation budget.
What would settle it
Take a discontinuous additive $f:\mathbb{R}\to\mathbb{R}$ (a Hamel-basis construction) and let $D$ be a point mass at some $p$ with $f(p)\neq g(p)$, where $g$ is the self-corrected function of (6)--(7). Run Algorithm 5; since a one-sided tester must accept every valid additive input with probability 1, any rejection -- in particular from Algorithm 6's third test, which can fail for such $f$ -- falsifies the one-sided additivity guarantee in Theorem 1.3.
Extended reading notes
Core claim
The paper's central claim is a doubly optimal result for linearity over $\mathbb{F}_2$ in the online-manipulation model: for every budget $t \le c\cdot\min\{\varepsilon^2, 1/n^2\}\cdot 2^n$ there is an $\varepsilon$-tester making $O(\max\{1/\varepsilon, \log t\})$ queries, and for $t \ge 20\varepsilon^2 2^n$ no $\varepsilon$-tester exists. The upper bound reuses [BKMR24]'s $k$-point test in the small-$t$ regime and switches to Goldreich--Ron sample-based testing in the large-$t$ regime, where the sample complexity $\Theta(n+1/\varepsilon)$ coincides with $\Theta(\log t + 1/\varepsilon)$. The impossibility extends Fischer's hard-distribution argument to show that any property with a YES instance and a constant-far NO instance becomes untestable once the adversary can manipulate an $\varepsilon$-fraction of the input within the tester's first $O(1/\varepsilon)$ queries. Over the reals, the paper claims a one-sided-error distribution-free $O(1/\varepsilon)$-query tester for additivity and for linearity of continuous functions, improving Fleming--Yoshida's $O((1/\varepsilon)\log(1/\varepsilon))$; the proof's insight is that a correct evaluation of the self-corrected function $g$ at a single witness point with constant probability is enough, since the other evaluations may be erroneous. The same repair removes the $\log(1/\varepsilon)$ factor from the degree-$d$ polynomial testers of [ABF+23].
Load-bearing premise
For the real additivity tester to keep one-sided error, every additive function must agree with its self-corrected version on the checked points; the paper only proves this for linear functions, and additive-but-not-linear functions can violate it.
Editorial extensions
If this is right
- Linearity over $\mathbb{F}_2$ is testable with $O(\log t + 1/\varepsilon)$ queries against budget-managing online adversaries for every $t \le c\cdot\min\{\varepsilon^2,1/n^2\}\cdot 2^n$, which is the largest manipulation budget for which testing is possible up to constants.
- Any property that has a YES instance and a constant-far NO instance becomes untestable in the online model once $t \ge (10/\alpha)\varepsilon^2 N$, so the impossibility is not special to linearity.
- Distribution-free additivity and linearity of continuous functions over $\mathbb{R}$ admit one-sided-error testers with $O(1/\varepsilon)$ queries, removing the $\log(1/\varepsilon)$ factor from the previous real-domain testers.
- The same $O(1/\varepsilon)$ query bound is obtained for distribution-free testing of degree-$d$ polynomials over the reals for constant $d$, including the approximate and rational-lattice variants.
Reading between the lines
- Beyond the paper, the regime-switch principle is likely portable: any property with an optimal sample-based tester and a $k$-point-style combination test should inherit a doubly optimal online tester with the same threshold $t\approx\Theta(\varepsilon^2 N)$.
- Beyond the paper, the observation that a constant-probability-correct self-correction suffices suggests that other algorithms amplifying an inner self-corrector to error $1-O(\varepsilon)$ only for a union bound can drop that amplification, potentially improving related distribution-free tests.
- A concrete scope check: if Theorem 1.3 is interpreted literally for additivity, it must accept all Hamel-basis additive functions; testing Algorithm 5 on a discontinuous additive function would settle whether the one-sided guarantee is real or only holds for linear inputs.
- The impossibility threshold $t=\Theta(\varepsilon^2 N)$ draws a clean line: below it linearity is testable with optimal queries, at it every property with a constant-far NO instance becomes untestable, so the online model offers no intermediate regime for such properties.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies linearity testing in two distinct settings. In the online manipulation model over F2 (Section 2), it presents a query-optimal tester resilient to t online manipulations per query for all t ≤ c·min{ε^2, 1/n^2}·2^n, combining a simplified k-point tester for small t and a sample-based tester for large t, and proves an impossibility result for t ≥ 20ε^2·2^n. In the distribution-free setting over the reals (Section 3), the paper claims O(1/ε)-query testers for additivity and for linearity of continuous functions, improving the Fleming-Yoshida tester, and extends the technique to degree-d polynomial testing with O(d^5 + d^2/ε) queries.
Significance. The online manipulation results are the strongest contribution: Theorem 1.1 gives a doubly optimal tester, and the sample-based tester observation (Theorem 2.6) is clean and useful. The impossibility result is a natural complement. The real-valued results, if correct, would be optimal and would simplify prior work. However, the additivity part of Theorem 1.3 has a serious gap in its one-sided error guarantee, and the linearity part is asserted without a complete proof. The low-degree tester (Theorem 1.4) appears sound and is a nice improvement. Overall, the paper contains significant and correct results in the online model, but the claims for additivity testing over the reals require major revision.
major comments (2)
- [§3.1, Theorem 1.3 (additivity claim)] The one-sided error guarantee for additivity is not established and, as stated, is incorrect. The completeness proof covers only linear functions: 'if f is linear, Algorithm 6 always accepts, and moreover, (i) g≡f and (ii) for all p,x∈R^n, g(p)=g_x(p), ensuring Algorithm 5 also accepts f.' Additive non-linear functions exist via Hamel bases. For such f, Eq. (6) gives g_x(p)=κ_p f(p/κ_p). When ||p||_2>1/50, κ_p=50||p||_2 is generally irrational, so f(p/κ_p) need not equal f(p)/κ_p, and hence g_x(p) need not equal f(p). If the unknown distribution D is a point mass at such a p, Algorithm 5 line 5 rejects with probability 1 on a valid additive input. Lemma 3.1 only guarantees that g is additive and that g_x(p) agrees with g(p) with probability >1/2; it does not imply g(p)=f(p). A separate completeness argument for all additive functions, or a change of the theorem statement to two-sided error, is required.
- [§3.1, Theorem 1.3 (linearity claim)] The linearity part of Theorem 1.3 is asserted but not proved. The text says 'the tester for linearity follows from the same arguments as in [FY20] where one uses our modified query-g subroutine in both their testers.' The additivity soundness proof relies on the fact that g is additive and f is ε-far from all additive functions, so that Pr_{p∼D}[f(p)≠g(p)]≥ε. For linearity, Lemma 3.1 only guarantees that g is additive, not linear, and f is only assumed ε-far from all linear functions, so the analogous step does not apply. The paper should either provide the precise FY20 argument adapted to the modified subroutine, or state explicitly which additional conditions (e.g., continuity of f) are used and how the constant-accuracy evaluation suffices.
minor comments (4)
- [§3.2, Claim 3.5] The sentence 'we have g_q(c_i p)≠g(c_i p) with probability most 1 − 1/(7d)' should read 'with probability at most 1/(7d)', or equivalently 'g_q(c_i p)=g(c_i p) with probability at least 1−1/(7d)'. The subsequent union bound uses the correct bound.
- [§2.2, Algorithm 3] The line 'Accept if span(X)≠{0,1}^n' is confusing, as it appears to accept when the sample does not span the whole space. The accompanying comment 'w.h.p. this doesn't happen' clarifies the intent, but the line should be rephrased and the role of this fallback acceptance should be explained.
- [§2, heading and text] The surname is misspelled as 'Kalmaj' in the Section 2 heading and in the text; the correct spelling is 'Kalemaj', matching the reference [KRV23].
- [§3.2, Theorems 3.6 and 3.7] These theorems are stated with proofs omitted. While they are said to follow from the same arguments as Theorem 1.4, the omission makes them difficult to verify, especially given the gap in Theorem 1.3. Please provide at least proof sketches.
Circularity Check
No circularity: the new results are proved from externally published lemmas; the only flagged issue in Theorem 1.3 is a completeness gap, not a derivation loop.
full rationale
At every load-bearing step the derivation imports an external, published theorem rather than assuming its own conclusion. Lemma 2.3 (BKMR24) supplies the k-point soundness used in Case I of Theorem 1.1; Lemma 2.5 (GR16) supplies the sample-based base tester for Case II; Theorem 2.6 is a new but self-contained reduction that does not presuppose testability; Theorem 2.8 adapts Fischer's external lower-bound argument; Lemma 3.1 (FY20) and Lemma 3.4 (ABF+23) supply the self-corrector guarantees for the real-valued and low-degree testers. That BKMR24 and ABF+23 share authors with the present paper is normal and not disqualifying: both are published theorems with assumptions that do not include the present target results, so they are independent evidence. The one genuine weakness is in the proof of Theorem 1.3: the completeness argument explicitly handles only 'if f is linear, Algorithm 6 always accepts, and moreover (i) g≡f ... ensuring Algorithm 5 also accepts f,' while the theorem promises one-sided error for all additive functions. For a Hamel-type additive non-linear f, the equality g(p)=f(p) is not established (and need not hold), so the one-sided-error guarantee for additivity is not proved as written. This is a correctness/completeness gap, not a circular reduction: the conclusion is not being fed back as an input, and no equation in the paper makes the additive claim equal to its premise by construction. Hence circularity score 0.
Assumptions & free parameters
assumptions (6)
- standard math Lemma 2.3: the k-point test rejects ε-far functions with probability at least min{1/4, kε/2}.
- domain assumption Lemma 3.1: if the characterization subroutine accepts with probability at least 1/10, the self-corrected function g is well-defined and additive, and each random opinion g_x(p) is correct with probability at least 1/2.
- standard math Lemma 3.4: for low-degree self-correction from ABF+23, if the characterization test fails with probability at most 2/3 then g is a degree-d polynomial and random opinions are correct with probability at least 1 - 1/(7d).
- standard math Fischer's lower bound argument: any tester for a property with a far pair induces a hard distribution and requires Ω(1/ε) queries.
- domain assumption The online manipulation model (Definition 2.1) with budget-managing adversaries as the threat model.
- domain assumption Non-linear additive functions over R exist and satisfy f(x+y)=f(x)+f(y) but not f(p/κ)=f(p)/κ for real κ.
Cite this review
Pith. "Pith review of On Optimal Testing of Linearity." pith.science (2026). https://pith.science/paper/RA6AJOLP
@misc{pith2026241114431,
author = {Pith},
title = {Pith review of: On Optimal Testing of Linearity},
year = {2026},
howpublished = {\url{https://pith.science/paper/RA6AJOLP}},
note = {Machine review of arXiv:2411.14431}
}
abstract
Linearity testing has been a focal problem in property testing of functions. We combine different known techniques and observations about linearity testing in order to resolve two recent versions of this task. First, we focus on the online manipulations model introduced by Kalemaj, Raskhodnikova and Varma (ITCS 2022 \& Theory of Computing 2023). In this model, up to $t$ data entries are adversarially manipulated after each query is answered. Ben-Eliezer, Kelman, Meir, and Raskhodnikova (ITCS 2024) showed an asymptotically optimal linearity tester that is resilient to $t$ manipulations per query, but their approach fails if $t$ is too large. We extend this result, showing an optimal tester for almost any possible value of $t$. First, we simplify their result when $t$ is small, and for larger values of $t$ we instead use sample-based testers, as defined by Goldreich and Ron (ACM Transactions on Computation Theory 2016). A key observation is that sample-based testing is resilient to online manipulations, but still achieves optimal query complexity for linearity when $t$ is large. We complement our result by showing that when $t$ is \emph{very} large, any reasonable property, and in particular linearity, cannot be tested at all. Second, we consider linearity over the reals with proximity parameter $\varepsilon$. Fleming and Yoshida (ITCS 2020) gave a tester using $O(1/\varepsilon\ \cdot log(1/\varepsilon))$ queries. We simplify their algorithms and modify the analysis accordingly, showing an optimal tester that only uses $O(1/\varepsilon)$ queries. This modification works for the low-degree testers presented in Arora, Bhattacharyya, Fleming, Kelman, and Yoshida (SODA 2023) as well, resulting in optimal testers for degree-$d$ polynomials, for any constant degree $d$.
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Reviewed August 12, 2026 · model on record in the stance chip above.
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