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Revision #1 to TR12-046 | 24th April 2012 21:28

A new upper bound on the query complexity for testing generalized Reed-Muller codes

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Revision #1
Authors: Madhu Sudan, Noga Ron-Zewi
Accepted on: 24th April 2012 21:28
Downloads: 885
Keywords: 


Abstract:

Over a finite field $\F_q$ the $(n,d,q)$-Reed-Muller code is the code given by evaluations of $n$-variate polynomials of total degree at most $d$ on all points (of $\F_q^n$). The task of testing if a function $f:\F_q^n \to \F_q$ is close to a codeword of an $(n,d,q)$-Reed-Muller code has been of central interest in complexity theory and property testing. The query complexity of this task is the minimal number of queries that a tester can make (minimum over all testers of the maximum number of queries over all random choices) while accepting all Reed-Muller codewords and rejecting words that are $\delta$-far from the code with probability $\Omega(\delta)$. (In this work we allow the constant in the $\Omega$ to depend on $d$.)

For codes over a prime field $\F_q$ the optimal query complexity is well-known and known to be $\Theta(q^{\lceil (d+1)/(q-1)\rceil})$, and the test consists of testing if $f$ is a degree $d$ polynomial on a randomly chosen $(\lceil (d+1)/(q-1) \rceil)$-dimensional affine subspace of $\F_q^n$. If $q$ is not a prime, then the above quantity remains a lower bound, whereas the previously known upper bound grows to $O(q^{\lceil (d+1)/(q - q/p) \rceil})$ where $p$ is the characteristic of the field $\F_q$. In this work we give a new upper bound of $(c q)^{(d+1)/q }$ on the query complexity, where $c$ is a universal constant. Thus for every $p$ and sufficiently large constant $q$ this bound improves over the previously known bound by a polynomial factor, as we let $d \to \infty$.

In the process we also give new upper bounds on the ``spanning weight'' of the dual of the Reed-Muller code (which is also a Reed-Muller code). The spanning weight of a code is the smallest integer $w$ such that codewords of Hamming weight at most $w$ span the code. The main technical contribution of this work is the design of tests that test a function by {\em not} querying its value on an entire subspace of the space, but rather on a carefully chosen (algebraically nice) subset of the points from low-dimensional subspaces.



Changes to previous version:

Clarified abstract. (Hopefully) fixed author's name.


Paper:

TR12-046 | 24th April 2012 17:11

A new upper bound on the query complexity for testing generalized Reed-Muller codes





TR12-046
Authors: Madhu Sudan, Noga Ron-Zewi
Publication: 24th April 2012 17:11
Downloads: 875
Keywords: 


Abstract:

Over a finite field $\F_q$ the $(n,d,q)$-Reed-Muller code is the code given by evaluations of $n$-variate polynomials of total degree at most $d$ on all points (of $\F_q^n$). The task of testing if a function $f:\F_q^n \to \F_q$ is close to a codeword of an $(n,d,q)$-Reed-Muller code has been of central interest in complexity theory and property testing. The query complexity of this task is the minimal number of queries that a tester can make (minimum over all testers of the maximum number of queries over all random choices) while accepting all Reed-Muller codewords and rejecting words that are $\delta$-far from the code with probability $\Omega(\delta)$. (In this work we allow the constant in the $\Omega$ to depend on $d$.)

For codes over a prime field $\F_q$ the optimal query complexity is well-known and known to be $\Theta(q^{\lceil (d+1)/(q-1)\rceil})$, and the test consists of testing if $f$ is a degree $d$ polynomial on a randomly chosen $(\lceil (d+1)/(q-1) \rceil)$-dimensional affine subspace of $\F_q^n$. If $q$ is not a prime, then the above quantity remains a lower bound, whereas the previously known upper bound grows to $O(q^{\lceil (d+1)/(q - q/p) \rceil})$ where $p$ is the characteristic of the field $\F_q$. In this work we give a new upper bound of $(c q)^{(d+1)/q }$ on the query complexity, where $c$ is a universal constant. Thus for every $p$ and sufficiently large $q$ this bound improves over the previously known bound by a polynomial factor.

In the process we also give new upper bounds on the ``spanning weight'' of the dual of the Reed-Muller code (which is also a Reed-Muller code). The spanning weight of a code is the smallest integer $w$ such that codewords of Hamming weight at most $w$ span the code. The main technical contribution of this work is the design of tests that test a function by {\em not} querying its value on an entire subspace of the space, but rather on a carefully chosen (algebraically nice) subset of the points from low-dimensional subspaces.



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