380 research outputs found
Expander Decomposition in Dynamic Streams
In this paper we initiate the study of expander decompositions of a graph G = (V, E) in the streaming model of computation. The goal is to find a partitioning ? of vertices V such that the subgraphs of G induced by the clusters C ? ? are good expanders, while the number of intercluster edges is small. Expander decompositions are classically constructed by a recursively applying balanced sparse cuts to the input graph. In this paper we give the first implementation of such a recursive sparsest cut process using small space in the dynamic streaming model.
Our main algorithmic tool is a new type of cut sparsifier that we refer to as a power cut sparsifier - it preserves cuts in any given vertex induced subgraph (or, any cluster in a fixed partition of V) to within a (?, ?)-multiplicative/additive error with high probability. The power cut sparsifier uses O?(n/??) space and edges, which we show is asymptotically tight up to polylogarithmic factors in n for constant ?
Non-Malleable Codes for Small-Depth Circuits
We construct efficient, unconditional non-malleable codes that are secure
against tampering functions computed by small-depth circuits. For
constant-depth circuits of polynomial size (i.e. tampering
functions), our codes have codeword length for a -bit
message. This is an exponential improvement of the previous best construction
due to Chattopadhyay and Li (STOC 2017), which had codeword length
. Our construction remains efficient for circuit depths as
large as (indeed, our codeword length remains
, and extending our result beyond this would require
separating from .
We obtain our codes via a new efficient non-malleable reduction from
small-depth tampering to split-state tampering. A novel aspect of our work is
the incorporation of techniques from unconditional derandomization into the
framework of non-malleable reductions. In particular, a key ingredient in our
analysis is a recent pseudorandom switching lemma of Trevisan and Xue (CCC
2013), a derandomization of the influential switching lemma from circuit
complexity; the randomness-efficiency of this switching lemma translates into
the rate-efficiency of our codes via our non-malleable reduction.Comment: 26 pages, 4 figure
Efficient Algorithms for Certifying Lower Bounds on the Discrepancy of Random Matrices
We initiate the study of the algorithmic problem of certifying lower bounds
on the discrepancy of random matrices: given an input matrix , output a value that is a lower bound on
for every , but
is close to the typical value of with high probability over
the choice of a random . This problem is important because of its
connections to conjecturally-hard average-case problems such as
negatively-spiked PCA, the number-balancing problem and refuting random
constraint satisfaction problems. We give the first polynomial-time algorithms
with non-trivial guarantees for two main settings. First, when the entries of
are i.i.d. standard Gaussians, it is known that with high probability. Our algorithm certifies that
with high probability. As an
application, this formally refutes a conjecture of Bandeira, Kunisky, and Wein
on the computational hardness of the detection problem in the negatively-spiked
Wishart model. Second, we consider the integer partitioning problem: given
uniformly random -bit integers , certify the non-existence
of a perfect partition, i.e. certify that for . Under the scaling , it is known that the
probability of the existence of a perfect partition undergoes a phase
transition from 1 to 0 at ; our algorithm certifies the
non-existence of perfect partitions for some . We also give
efficient non-deterministic algorithms with significantly improved guarantees.
Our algorithms involve a reduction to the Shortest Vector Problem.Comment: ITCS 202
Learning Reserve Prices in Second-Price Auctions
This paper proves the tight sample complexity of Second-Price Auction with
Anonymous Reserve, up to a logarithmic factor, for all value distribution
families that have been considered in the literature. Compared to Myerson
Auction, whose sample complexity was settled very recently in (Guo, Huang and
Zhang, STOC 2019), Anonymous Reserve requires much fewer samples for learning.
We follow a similar framework as the Guo-Huang-Zhang work, but replace their
information theoretical argument with a direct proof
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