arXiv subjects

Rishi Saket

Publications and source records attributed to Rishi Saket.

22 records · Page 2Linked to original sources

arXiv2013

Hardness of Finding Independent Sets in 2-Colorable and Almost 2-Colorable Hypergraphs

This work studies the hardness of finding independent sets in hypergraphs which are either 2-colorable or are almost 2-colorable, i.e. can be 2-colored after removing a small fraction of vertices and the incident hyperedges. To be precise, say that a hypergraph is (1-eps)-almost 2-colorable if removing an eps fraction of its vertices and all hyperedges incident on them makes the remaining hypergraph 2-colorable. In particular we prove the following results. For an arbitrarily small constant gamma > 0, there is a constant xi > 0, such that, given a 4-uniform hypergraph on n vertices which is (1 - eps)-almost 2-colorable for eps = 2^{-(log n)^xi}, it is quasi-NP-hard to find an independent set of n/(2^{(log n)^{1-gamma}}) vertices. For any constants eps, delta > 0, given as input a 3-uniform hypergraph on $n$ vertices which is (1-eps)-almost 2-colorable, it is NP-hard to find an independent set of delta n vertices. Assuming the d-to-1 Games Conjecture the following holds. For any constant delta > 0, given a 2-colorable 3-uniform hypergraph on n vertices, it is NP-hard to find an independent set of delta n vertices. The hardness result on independent set in almost 2-colorable 3-uniform hypergraphs was earlier known only assuming the Unique Games Conjecture. In this work we prove the result unconditionally. For independent sets in 2-colorable 3-uniform hypergaphs we prove the first strong hardness result, albeit assuming the d-to-1 Games Conjecture. Our result on almost 2-colorable 4-uniform hypergraphs gives the first nearly polynomial hardness factor for independent set in hypergraphs which are (almost) colorable with constantly many colors. It partially bridges the gap between the previous best lower bound of poly(log n) and the algorithmic upper bounds of n^{Omega(1)}.

cs.CC↗

arXiv2013

A PTAS for the Classical Ising Spin Glass Problem on the Chimera Graph Structure

We present a polynomial time approximation scheme (PTAS) for the minimum value of the classical Ising Hamiltonian with linear terms on the Chimera graph structure as defined in the recent work of McGeoch and Wang. The result follows from a direct application of the techniques used by Bansal, Bravyi and Terhal who gave a PTAS for the same problem on planar and, in particular, grid graphs. We also show that on Chimera graphs, the trivial lower bound is within a constant factor of the optimum.

cs.DS↗

arXiv2012

Stochastic Vehicle Routing with Recourse

We study the classic Vehicle Routing Problem in the setting of stochastic optimization with recourse. StochVRP is a two-stage optimization problem, where demand is satisfied using two routes: fixed and recourse. The fixed route is computed using only a demand distribution. Then after observing the demand instantiations, a recourse route is computed -- but costs here become more expensive by a factor lambda. We present an O(log^2 n log(n lambda))-approximation algorithm for this stochastic routing problem, under arbitrary distributions. The main idea in this result is relating StochVRP to a special case of submodular orienteering, called knapsack rank-function orienteering. We also give a better approximation ratio for knapsack rank-function orienteering than what follows from prior work. Finally, we provide a Unique Games Conjecture based omega(1) hardness of approximation for StochVRP, even on star-like metrics on which our algorithm achieves a logarithmic approximation.

cs.DS↗

arXiv2011

Nearly Optimal NP-Hardness of Vertex Cover on k-Uniform k-Partite Hypergraphs

We study the problem of computing the minimum vertex cover on k-uniform k-partite hypergraphs when the k-partition is given. On bipartite graphs (k = 2), the minimum vertex cover can be computed in polynomial time. For general k, the problem was studied by Lovász, who gave a k/2 -approximation based on the standard LP relaxation. Subsequent work by Aharoni, Holzman and Krivelevich showed a tight integrality gap of (k/2 - o(1)) for the LP relaxation. While this problem was known to be NP-hard for k >= 3, the first non-trivial NP-hardness of approximation factor of k/4- \eps was shown in a recent work by Guruswami and Saket. They also showed that assuming Khot's Unique Games Conjecture yields a k/2 - \eps inapproximability for this problem, implying the optimality of Lovász's result. In this work, we show that this problem is NP-hard to approximate within k/2- 1 + 1/2k -\eps. This hardness factor is off from the optimal by an additive constant of at most 1 for k >= 4. Our reduction relies on the Multi-Layered PCP of Dinur et al. and uses a gadget - based on biased Long Codes - adapted from the LP integrality gap of Aharoni et al. The nature of our reduction requires the analysis of several Long Codes with different biases, for which we prove structural properties of the so called cross-intersecting collections of set families - variants of which have been studied in extremal set theory.

cs.CC↗