~~Five~~ Six papers this month: results on testing separations, linearity testing in \(\mathbb{R}^n\), testing for regular languages, graph property testing, topological property testing, and Boolean rank.

**Hard properties with (very) short PCPPs and their applications**, by Omri Ben-Eliezer, Eldar Fischer, Amit Levi, and Ron D. Rothblum (arXiv). Probably, the most significant takeaway from this work is a (largest possible) separation between standard and tolerant property testing. PCPPs (Probabilistically Checkable Proofs of Proximity) are the “NP” variant of property testing, where the tester is aided by a proof string. Consider property \(\mathcal{P}\). If \(x \in \mathcal{P}\), there must be a proof string that makes the tester accept (with probability 1). If \(x\) is far from \(\mathcal{P}\) (in the usual property testing sense), for any proof string, the tester must reject with sufficiently high probability. PCPPs have played a role in the classical constructions of PCPs, but have also found uses in getting a better understanding of property testing itself. And this paper shows how PCPP constructions can be used to get property testing separations. The main result in this paper is a property \(\mathcal{P}\) that (basically) requires \(\Omega(n)\) queries to “property test”, but has a PCPP system where the proof length is \(\widetilde{O}(n)\). (\(n\) is the input length.) The main construction uses collections of random linear codes. Significantly, these constructions show a strong separation between standard vs tolerant property testing, and standard vs erasure-resilient property testing. (The latter is a recent variant by Dixit et al, where certain parts of the input are hidden from the tester.) There is a property that is testable in a constant number of queries, but requires \(\widetilde{\Omega}(n)\) queries to test tolerantly (for any non-trivial choice of parameters). An analogous result holds for erasure-resilient testing.

**Distribution-Free Testing of Linear Functions on R^n**, by Noah Fleming and Yuichi Yoshida (arXiv). Linearity testing is arguably *the* canonical problem in property testing, yet there is still much to be learned about it. This paper considers functions \(f: \mathbb{R}^n \to \mathbb{R}\), and the *distribution-free setting*. (In this setting, distance is measured according is an unknown distribution \(\mathcal{D}\) over the input, and the tester can access samples from this distribution. For \(\mathbb{R}^n\), the “standard” distribution would the \(n\)-dimensional Gaussian.) The main result is that linearity testing can be done in the distribution-free setting with \(\widetilde{O}(1/\varepsilon)\) queries, assuming that the distribution is continuous. The primary technical tool, an interesting result in its own right, is that additivity \((f(x+y) = f(x) + f(y))\) can be tested in \(\widetilde{O}(1/\varepsilon)\) queries. The significance of the testing result is cemented by an \(\Omega(n)\) lower bound for sample-based testers.

**Sliding window property testing for regular languages** by Moses Ganardi, Danny Hucke, Markus Lohrey, Tatiana Starikovskaya (arXiv). Fix a regular language \(\mathcal{R}\). Consider the streaming model, and the basic question of recognizing whether the string (being streamed) is in \(\mathcal{R}\). Simple, you will say! Run the DFA recognizing \(\mathcal{R}\) in constant space. Now, suppose there is a sliding window length of \(n\). The aim is to determine if the past \(n\) symbols (the “active window”) form a string in \(\mathcal{R}\). Suprisingly (at least to me), there is a full characterization of the space required for randomized algorithms, and (depending on \(\mathcal{R}\)), it is either \(\Theta(1)\), \(\Theta(\log\log n)\), \(\Theta(\log n)\), or \(\Theta(n)\). In the interest of beating these lower bounds, suppose we wish to property test on the active window. It turns out the answer is quite nuanced. There are deterministic \(O(\log n)\)-space testers and randomized two-sided \(O(1/\varepsilon)\)-space testers for all regular languages. For randomized one-sided testers, there are multiple possibilities for the optimal space complexity, and there is a full characterization of these regular languages.

**A characterization of graph properties testable for general planar graphs with one-sided error (It’s all about forbidden subgraphs)** by Artur Czumaj and Christian Sohler (arXiv). Property testing of sparse graphs has been receiving more attention, but most results focus on the bounded degree setting. Unfortunately, many of these results break quite dramatically on sparse graphs with unbounded degrees. This paper focuses on property testing, within the class of unbounded degree planar graphs. (Meaning, the input is always assumed to be planar.) The results achieve a significant goal: as the title suggests, there is a complete characterization of properties that are constant-query testable with one-sided error. The easier part is in showing that all such properties can be reduced to testing \(H\)-freeness. The harder (remarkable) result is \(H\)-freeness can be tested in general planar queries with constant queries. (This is non-trivial even for triangle-freeness.) And, as is easy to conjecture but hard to prove, these results carry over for all minor-closed families. As a small indication of the challenge, most testers for bounded-degree graphs work by doing constant depth BFSes. When high degree vertices are present, this method fails, and we really need new ideas to deal with such graphs.

**Near Coverings and Cosystolic Expansion – an example of topological property testing** by Irit Dinur and Roy Meshulam (ECCC). In most algebraic settings, property testing results can be seen as local to global theorems. When do local constraints on a large object imply a global condition? This paper gives a topological instantiation of this phenomenon. We need to define the *cover* of a simplicial complex \(X\). For concreteness, think of a 2D simplicial complex \(X\), which is a hypergraph with hyperedges of size at most 3, where subsets of hyperedges are also present. A 2-cover is a simplicial complex \(X’\) with the following property. It has two copies of each vertex of \(X\). Each hyperedge of \(X\) must have two “corresponding” disjoint copies in \(X’\). Let the copies of vertex \(v\) be \(v_0, v_1\). Then, for every hyperedge (say) \((u,v,w)\) of \(X\), there must be two disjoint hyperedges in \(X’\) involving copies of the corresponding vertices. One can consider the property testing twist: if the neighborhoods of “most” vertices \(v\) in \(X\) satisfy these condition (with respect to the neighborhoods of the copies of \(v\) in \(X’\)), then is \(X’\) close to being a cover of \(X\)? Indeed, this paper proves that such a “property testing condition” holds iff \(X\) is a high-dimensional expander.

**Property testing of the Boolean and binary rank** by Michal Parnas, Dana Ron, and Adi Shraibman (arXiv). The Boolean rank of a matrix \(M\) is a fundamental quantity that appears in many lower bound constructions. (Recall that an \(n \times m\) Boolean matrix \(M\) has a rank \(r\) if \(M\) can be expressed as \(X \cdot Y\), where \(X \in \mathbb{F}_2^{n \times d}\) and \(Y \in \mathbb{F}_2^{d \times m}\).) In the real-valued setting, results show that one can property test rank in \(poly(d/\varepsilon)\) queries. This paper proves an analogous result for the Boolean rank. There is a surprise element here: over reals, the rank can be computed in polynomial time, and many of the geometric intuitions can be brought over to the property testing problem. On the other hand, the Boolean rank is NP-hard to compute exactly, yet we can still get a tester with \(poly(d)\) query complexity. The paper also gives results for *binary rank*. For the binary rank, we require the component matrices \(X, Y\) to be Boolean, but algebraic operations are over the reals. In the case, the tester has query complexity \(2^{2d}\) (with varying dependencies on \(\varepsilon\) for adaptive/non-adaptive testers). The intriguing open problem is whether \(poly(d)\)-query testers exist for binary rank.