12 research outputs found
The Complexity of Power-Index Comparison
We study the complexity of the following problem: Given two weighted voting
games G' and G'' that each contain a player p, in which of these games is p's
power index value higher? We study this problem with respect to both the
Shapley-Shubik power index [SS54] and the Banzhaf power index [Ban65,DS79]. Our
main result is that for both of these power indices the problem is complete for
probabilistic polynomial time (i.e., is PP-complete). We apply our results to
partially resolve some recently proposed problems regarding the complexity of
weighted voting games. We also study the complexity of the raw Shapley-Shubik
power index. Deng and Papadimitriou [DP94] showed that the raw Shapley-Shubik
power index is #P-metric-complete. We strengthen this by showing that the raw
Shapley-Shubik power index is many-one complete for #P. And our strengthening
cannot possibly be further improved to parsimonious completeness, since we
observe that, in contrast with the raw Banzhaf power index, the raw
Shapley-Shubik power index is not #P-parsimonious-complete.Comment: 12 page
Self-Specifying Machines
We study the computational power of machines that specify their own
acceptance types, and show that they accept exactly the languages that
\manyonesharp-reduce to NP sets. A natural variant accepts exactly the
languages that \manyonesharp-reduce to P sets. We show that these two classes
coincide if and only if \psone = \psnnoplusbigohone, where the latter class
denotes the sets acceptable via at most one question to \sharpp followed by
at most a constant number of questions to \np.Comment: 15 pages, to appear in IJFC
Nondeterministic functions and the existence of optimal proof systems
We provide new characterizations of two previously studied questions on nondeterministic function classes: Q1: Do nondeterministic functions admit efficient deterministic refinements? Q2: Do nondeterministic function classes contain complete functions? We show that Q1 for the class is equivalent to the question whether the standard proof system for SAT is p-optimal, and to the assumption that every optimal proof system is p-optimal. Assuming only the existence of a p-optimal proof system for SAT, we show that every set with an optimal proof system has a p-optimal proof system. Under the latter assumption, we also obtain a positive answer to Q2 for the class . An alternative view on nondeterministic functions is provided by disjoint sets and tuples. We pursue this approach for disjoint -pairs and its generalizations to tuples of sets from and with disjointness conditions of varying strength. In this way, we obtain new characterizations of Q2 for the class . Question Q1 for is equivalent to the question of whether every disjoint -pair is easy to separate. In addition, we characterize this problem by the question of whether every propositional proof system has the effective interpolation property. Again, these interpolation properties are intimately connected to disjoint -pairs, and we show how different interpolation properties can be modeled by -pairs associated with the underlying proof system
A Superpolynomial Lower Bound on the Size of Uniform Non-constant-depth Threshold Circuits for the Permanent
We show that the permanent cannot be computed by DLOGTIME-uniform threshold
or arithmetic circuits of depth o(log log n) and polynomial size.Comment: 11 page
Possible Winners in Noisy Elections
We consider the problem of predicting winners in elections, for the case
where we are given complete knowledge about all possible candidates, all
possible voters (together with their preferences), but where it is uncertain
either which candidates exactly register for the election or which voters cast
their votes. Under reasonable assumptions, our problems reduce to counting
variants of election control problems. We either give polynomial-time
algorithms or prove #P-completeness results for counting variants of control by
adding/deleting candidates/voters for Plurality, k-Approval, Approval,
Condorcet, and Maximin voting rules. We consider both the general case, where
voters' preferences are unrestricted, and the case where voters' preferences
are single-peaked.Comment: 34 page
Parity Separation: A Scientifically Proven Method for Permanent Weight Loss
Given an edge-weighted graph G, let PerfMatch(G) denote the weighted sum over
all perfect matchings M in G, weighting each matching M by the product of
weights of edges in M. If G is unweighted, this plainly counts the perfect
matchings of G.
In this paper, we introduce parity separation, a new method for reducing
PerfMatch to unweighted instances: For graphs G with edge-weights -1 and 1, we
construct two unweighted graphs G1 and G2 such that PerfMatch(G) =
PerfMatch(G1) - PerfMatch(G2). This yields a novel weight removal technique for
counting perfect matchings, in addition to those known from classical
#P-hardness proofs. We derive the following applications:
1. An alternative #P-completeness proof for counting unweighted perfect
matchings.
2. C=P-completeness for deciding whether two given unweighted graphs have the
same number of perfect matchings. To the best of our knowledge, this is the
first C=P-completeness result for the "equality-testing version" of any natural
counting problem that is not already #P-hard under parsimonious reductions.
3. An alternative tight lower bound for counting unweighted perfect matchings
under the counting exponential-time hypothesis #ETH.
Our technique is based upon matchgates and the Holant framework. To make our
#P-hardness proof self-contained, we also apply matchgates for an alternative
#P-hardness proof of PerfMatch on graphs with edge-weights -1 and 1.Comment: 14 page
A Dichotomy Theorem for Polynomial Evaluation
A dichotomy theorem for counting problems due to Creignou and Hermann states
that or any nite set S of logical relations, the counting problem #SAT(S) is
either in FP, or #P-complete. In the present paper we show a dichotomy theorem
for polynomial evaluation. That is, we show that for a given set S, either
there exists a VNP-complete family of polynomials associated to S, or the
associated families of polynomials are all in VP. We give a concise
characterization of the sets S that give rise to "easy" and "hard" polynomials.
We also prove that several problems which were known to be #P-complete under
Turing reductions only are in fact #P-complete under many-one reductions
Parallel Polynomial Permanent Mod Powers of 2 and Shortest Disjoint Cycles
We present a parallel algorithm for permanent mod 2^k of a matrix of
univariate integer polynomials. It places the problem in ParityL subset of
NC^2. This extends the techniques of [Valiant], [Braverman, Kulkarni, Roy] and
[Bj\"orklund, Husfeldt], and yields a (randomized) parallel algorithm for
shortest 2-disjoint paths improving upon the recent result from (randomized)
polynomial time.
We also recognize the disjoint paths problem as a special case of finding
disjoint cycles, and present (randomized) parallel algorithms for finding a
shortest cycle and shortest 2-disjoint cycles passing through any given fixed
number of vertices or edges
A Superpolynomial Lower Bound on the Size of Uniform Non-constant-depth Threshold Circuits for the Permanent
11 pagesWe show that the permanent cannot be computed by DLOGTIME-uniform threshold or arithmetic circuits of depth o(log log n) and polynomial size