Chicago Journal of Theoretical Computer Science

Volume 2015 Abstracts

  1. Unique Games on the Hypercube by Naman Agarwal, Guy Kindler, Alexandra Kolla, and Luca Trevisan
    February 6, 2015.

    In this paper, we investigate the validity of the Unique Games Conjecture when the constraint graph is the boolean hypercube. We construct an almost optimal integrality gap instance on the Hypercube for the Goemans-Williamson semidefinite program (SDP) for Max-2-LIN$(Z_2)$. We conjecture that adding triangle inequalities to the SDP provides a polynomial time algorithm to solve Unique Games on the hypercube.

  2. Entropy of weight distributions of small-bias spaces and pseudobinomiality by Louay Bazzi
    July 10, 2015.

    A classical bound in information theory asserts that small $L_1$-distance between probability distributions implies small difference in Shannon entropy, but the converse need not be true. We show that if a probability distribution on $\{0,1\}^n$ has small-bias, then the converse holds for its weight distribution in the proximity of the binomial distribution. Namely, we argue that if a probability distribution $\mu$ on $\{0,1\}^n$ is $\delta$-biased, then $\| \overline{\mu} -\ bin_n \|_1^2 \leq (2\ln{2})(n \delta + H(bin_n) - H(\overline{\mu}))$, where $\overline{\mu}$ is the weight distribution of $\mu$ and $bin_n$ is the binomial distribution on $\{0,\ldots, n\}$. The key result behind this bound is a lemma which asserts the non-positivity of all the Fourier coefficients of the log-binomial function $L:\{0,1\}^n \rightarrow {\mathbf R}$ given by $L(x) = \lg{bin_n(|x|) }$. The original question which motivated the work reported in this paper is the problem of explicitly constructing a small subset of $\{0,1\}^n$ which is $\epsilon$-pseudobinomial in the sense that the weight distribution of each of its restrictions and translations is $\epsilon$-close to the binomial distribution. We study the notion of pseudobinomiality and we conclude that, for spaces with $n^{-\Theta(1)}$-small bias, the pseudobinomiality error in the $L_1$-sense is equivalent to that in the entropy-difference-sense, in the $n^{-\Theta(1)}$-error regimen. We also study the notion of average case pseudobinomiality, and we show that for spaces with $n^{-\Theta(1)}$-small bias, the average entropy of the weight distribution of a random translation of the space is $n^{-\Theta(1)}$-close to the entropy of the binomial distribution. We discuss resulting questions on the pseudobinomiality of sums of independent small-bias spaces. Using the above results, we show that the following conjectures are equivalent: (1) For all independent $\delta$-biased random vectors $X,Y\in \{0,1\}^n$, the ${\mathbf F}_2$-sum $X+Y$ is $O((n\delta)^{\Theta(1)})$-pseudobinomial; (2) For all independent $\delta$-biased random vectors $X,Y\in \{0,1\}^n$, the entropy of the weight of the sum $H(|X+Y|)\geq \min\{H(|X|),H(|Y|)\} - O((n\delta)^{\Theta(1)}).$ <\li>

  3. Reducing uniformity in Khot-Saket hypergraph coloring hardness reductions by Girish Varma
    July 26, 2015.

    In a recent result, Khot and Saket [FOCS 2014] proved the quasi-NP-hardness of coloring a $2$-colorable $12$-uniform hypergraph with $2^{{(\log n)}^{\Omega(1)}}$ colors. This result was proved using a novel outer PCP verifier which had a strong soundness guarantee. We reduce the arity in their result by modifying their 12-query inner verifier to an 8-query inner verifier based on the hypergraph coloring hardness reductions of Guruswami et al [STOC 2014]. More precisely, we prove quasi-NP-hardness of the following problems on $n$-vertex hypergraphs.

  4. Noise Stable Halfspaces are Close to Very Small Juntas by Ilias Diakonikolas, Ragesh Jaiswal, Rocco A. Servedio, Li-Yang Tan, and Andrew Tan
    October 19, 2015.

    Bourgain showed that any noise stable Boolean function $f$ can be well-approximated by a junta. In this note we give an exponential sharpening of the parameters of Bourgain's result under the additional assumption that $f$ is a halfspace.

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Janos Simon