# Open problem for Jan 2014: Explicit lower bound for MAPs

The open problem of the month is a new feature on PTReview. Every month we put up on open problem contributed by our readers. To know more about getting your problem broadcasted, check out the link “About Open Problem of the Month” at the top of the page.

The first ever open problem post is by Tom Gur on Merlin-Arthur proofs of proximity. This post doubles up a great survey on this new topic. Thanks Tom!

Primary reference: Non-interactive proofs of proximity by Gur and Rothblum, ECCC, 2013.

Basic setting: Recent work by Ron Rothblum and myself [GR13] initiates the study of $$\mathsf{MA}$$ proof-system analogues of property testing, referred to as $$\mathsf{MA}$$ proofs of proximity ($$\mathsf{MAP}$$s). A proof-aided tester for property $$\Pi$$ is given oracle access to an input $$x$$ and free access to a proof $$w$$. A valid $$\mathsf{MAP}$$ demands the following property. Given a proximity parameter $$\epsilon>0$$, for inputs $$x \in \Pi$$, there exists a proof that the tester accepts with high probability, and for inputs $$x$$ that are $$\epsilon$$-far from $$\Pi$$ no proof will make the tester accept, except with some small probability of error.

Open problem: Can one show an explicit property where $$\mathsf{MAP}$$s do not give additional power over standard testers? Formally, show an explicit property that requires any $$\mathsf{MAP}$$ to make $$\Omega(|x|)$$ queries even when given a long (e.g., length $$|x|/100$$) proof?

Background: Goldreich, Goldwasser, and Ron [GGR98] showed that “almost all” properties require probing a constant fraction of the tested object. This limitation was the primary motivation for $$\mathsf{MAP}$$s. Observe that given a sufficiently long proof, every property can be tested very efficiently by an $$\mathsf{MAP}$$. Consider a proof that fully describes the object. The tester has free access to the proof, so all it has to do is verify the consistency of the proof and the object (which can be done by only making $$O(1/\epsilon)$$ random queries). The tester can check for free that the object described in the proof has the desired property.

Indeed, the name of the game is to construct $$\mathsf{MAP}$$s with short proofs and significantly lower query complexity than standard testers. There exists a property that has an $$\mathsf{MAP}$$ using a logarithmic-length proof and only a constant number of queries, but requires nearly linear number of queries for standard testing (see Section 3 of [GR13]).

Nevertheless, $$\mathsf{MAP}$$s with short proofs are far from being omnipotent. A random property requires any $$\mathsf{MAP}$$ to perform $$\Omega(n)$$ queries (where $$n$$ is the length of the input), even when given a $$n/100$$ length proof (Section 5 of [GR13]). The open problem is to find an explicit property that matches this lower bound. The best lower bound on explicit properties is far weaker; specifically, Fischer, Goldhirsh, and Lachish [FGL13] showed that testing a certain family of linear codes (namely, codes with “large” dual distance) requires any $$\mathsf{MAP}$$ with a proof of length $$p \ge 1$$ to make $$\Omega(n/p)$$ queries.

References

[FGL13] E. Fischer, Y. Goldhirsh, and O. Lachish. Partial tests, universal tests and decomposability. Innovations in Theoretical Computer Science (ITCS), 2013.

[GGR98] O. Goldreich, S. Goldwasser, and D. Ron. Property testing and its connection to learning and approximation. Journal of the ACM (JACM), 1998.

[GR13] T. Gur and R. Rothblum. Non-interactive proofs of proximity. Electronic Colloquium on Computational Complexity (ECCC), 2013.