Improved Low-Degree Testing and its Applications

NP = PCP(logn, 1) and related results crucially depend upon the close connection between the probability with which a function passes a low degree test and the distance of this function to the nearest degree d polynomial. In this paper we study a test proposed by Rubinfeld and Sudan [30]. The strongest previously known connection for this test states that a function passes the test with probability δ for some δ > 7/8 iff the function has agreement ≈ δ with a polynomial of degree d. We present a new, and surprisingly strong, analysis which shows that the preceding statement is true for arbitrarily small δ, provided the field size is polynomially larger than d/δ. The analysis uses a version of Hilbert irreducibility, a tool of algebraic geometry. As a consequence we obtain an alternate construction for the following proof system: A constant prover 1-round proof system for NP languages in which the verifier uses O(logn) random bits, receives answers of size O(logn) bits, and has an error probability of at most 2− log 1− . Such a proof system, which implies the NP-hardness of approximating Set Cover to within Ω(logn) factors, has already been obtained by Raz and Safra [29]. Raz and Safra obtain their result by giving a strong analysis, in the sense described above, of a new low-degree test that they present. A second consequence of our analysis is a self tester/corrector for any buggy program that (supposedly) computes a polynomial over a finite field. If the program is correct only on δ fraction of inputs where δ = 1/ |F| 0.5, then the tester/corrector determines δ and generates O( 1 δ ) values for every input, such that one of them is the correct output. In fact, our results yield something stronger: Given the buggy program, we can construct O( 1 δ ) randomized programs such that one of them is correct on every input, with high probability. Such a strong selfcorrector is a useful tool in complexity theory with some applications known. ∗arora@cs.princeton.edu. Supported by an NSF CAREER award, an Alfred P. Sloan Fellowship, and a Packard Fellowship. †madhu@lcs.mit.edu. Laboratory for Computer Science, MIT, Cambridge, MA 02139. Part of this work was done when this author was at the IBM Thomas J. Watson Research Center.

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