On ACC

We show that every languageL in the class ACC can be recognized by depth-two deterministic circuits with a symmetric-function gate at the root and AND gates of fan-in log ^O(1) n at the leaves, or equivalently, there exist polynomialsp _n (x ₁ ,..., x _n ) overZ of degree log ^O(1) n and with coefficients of magnitude and functionsh _n :Z{0,1} such that for eachn and eachx{0,1} ⁿ ,XL ^(x) =h _n (p _n (x ₁ ,...,x _n )). This improves an earlier result of Yao (1985). We also analyze and improve modulus-amplifying polynomials constructed by Toda (1991) and Yao (1985).     

Liouvillian solutions of third order differential equations

The Kovacic algorithm and its improvements give explicit formulae for the Liouvillian solutions of second order linear differential equations. Algorithms for third order differential equations also exist, but the tools they use are more sophisticated and the computations more involved. In this paper we refine parts of the algorithm to find Liouvillian solutions of third order equations. We show that, except for four finite groups and a reduction to the second order case, it is possible to give a formula in the imprimitive case. We also give necessary conditions and several simplifications for the computation of the minimal polynomial for the remaining finite set of finite groups (or any known finite group) by extracting ramification information from the character table. Several examples have been constructed, illustrating the possibilities and limitations.     

Exact learning of DNF formulas using DNF hypotheses

We show the following: (a) For any ε>0, log^(3+ε)n-term DNF cannot be polynomial-query learned with membership and strongly proper equivalence queries. (b) For sufficiently large t, t-term DNF formulas cannot be polynomial-query learned with membership and equivalence queries that use t^1+ε-term DNF formulas as hypotheses, for some ε<1 (c) Read-thrice DNF formulas are not polynomial-query learnable with membership and proper equivalence queries. (d) logn-term DNF formulas can be polynomial-query learned with membership and proper equivalence queries. (This complements a result of Bshouty, Goldman, Hancock, and Matar that -term DNF can be so learned in polynomial time.)Versions of (a)-(c) were known previously, but the previous versions applied to polynomial-time learning and used complexity theoretic assumptions. In contrast, (a)-(c) apply to polynomial-query learning, imply the results for polynomial-time learning, and do not use any complexity-theoretic assumptions.     

The computational complexity of recognizing permutation functions

Let be a finite field withq elements and a rational function over . No polynomial-time deterministic algorithm is known for the problem of deciding whetherf induces a permutation on . The problem has been shown to be in co-R co-NP, and in this paper we prove that it is inR NP and hence inZPP, and it is deterministic polynomial-time reducible to the problem of factoring univariate polynomials over . Besides the problem of recognizing prime numbers, it seems to be the only natural decision problem inZPP unknown to be inP. A deterministic test and a simple probabilistic test for permutation functions are also presented.     

Circuits constructed with MOD q gates cannot compute “AND” in sublinear size

Algebraic techniques are used to prove that any circuit constructed with MOD _q gates that computes the AND function must use (n) gates at the first level. The best bound previously known to be valid for arbitraryq was (logn).     

Towards optimal simulations of formulas by bounded-width programs

We show that, over an arbitrary ring, for any fixed >0, all balanced algebraic formulas of sizes are computed by algebraic straight-line programs that employ a constant number of registers and have lengthO (s ¹⁺). In particular, in the special case where the ring isGF(2), we obtain a technique for simulating balanced Boolean formulas of sizes by bounded-width branching programs of lengthO(s ¹⁺), for any fixed >0. This is an asymptotic improvement in efficiency over previous simulations in both the Boolean and algebraic settings.     

Non-deterministic exponential time has two-prover interactive protocols

We determine the exact power of two-prover interactive proof systems introduced by Ben-Or, Goldwasser, Kilian, and Wigderson (1988). In this system, two all-powerful noncommunicating provers convince a randomizing polynomial time verifier in polynomial time that the inputx belongs to the languageL. We show that the class of languages having tow-prover interactive proof systems is nondeterministic exponential time.We also show that to prove membership in languages inEXP, the honest provers need the power ofEXP only.The first part of the proof of the main result extends recent techniques of polynomial extrapolation used in the single prover case by Lund, Fortnow, Karloff, Nisan, and Shamir.The second part is averification scheme for multilinearity of a function in several variables held by an oracle and can be viewed as an independent result onprogram verification. Its proof rests on combinatorial techniques employing a simple isoperimetric inequality for certain graphs:     

The complexity of computing maximal word functions

Maximal word functions occur in data retrieval applications and have connections with ranking problems, which in turn were first investigated in relation to data compression [21]. By the maximal word function of a languageL ^*, we mean the problem of finding, on inputx, the lexicographically largest word belonging toL that is smaller than or equal tox.In this paper we present a parallel algorithm for computing maximal word functions for languages recognized by one-way nondeterministic auxiliary pushdown automata (and hence for the class of context-free languages).This paper is a continuation of a stream of research focusing on the problem of identifying properties others than membership which are easily computable for certain classes of languages. For a survey, see [24].     

Polynomial-time compression

This paper studies the class ofinfinite sets that have minimal perfect hash functions—one-to-one onto maps between the sets and ^*-computable in polynomial time. We will call such sets P-compressible. We show that all standard NP-complete sets are P-compressible, and give a structural condition,E = ₂ ^E , sufficient to ensure thatall infinite NP sets are P-compressible. On the other hand, we present evidence that some infinite NP sets, and indeed some infinite P sets, are not P-compressible: if an infinite NP setA is P-compressible, thenA has an infinite sparse NP subset, yet we construct a relativized world in which some infinite NP sets lack infinite sparse NP subsets. This world is built upon a result that is of interest in its own right; we determine optimally—with respect to any relativizable proof technique—the complexity of the easiest infinite sparse subsets that infinite P sets are guaranteed to have.     

The complexity of the max word problem and the power of one-way interactive proof systems

We study the complexity of the max word problem for matrices, a variation of the well-known word problem for matrices. We show that the problem is NP-complete, and cannot be approximated within any constant factor, unless P=NP. We describe applications of this result to probabilistic finite state automata, rational series andk-regular sequences. Our proof is novel in that it employs the theory of interactive proof systems, rather than a standard reduction argument. As another consequence of our results, we characterize NP exactly in terms ofone-way interactive proof systems.     

NL-printable sets and nondeterministic Kolmogorov complexity

P-printable sets were defined by Hartmanis and Yesha and have been investigated by several researchers. The analogous notion of L-printable sets was defined by Fortnow et al.; both P-printability and L-printability were shown to be related to notions of resource-bounded Kolmogorov complexity. Nondeterministic logspace (NL)-printability was defined by Jenner and Kirsig, but some basic questions regarding this notion were left open. In this paper we answer a question of Jenner and Kirsig by providing a machine-based characterization of the NL-printable sets.     

Weakly complete problems are not rare

Certain natural decision problems are known to be intractable because they are complete for E, the class of all problems decidable in exponential time. Lutz recently conjectured that many other seemingly intractable problems are not complete for E, but are intractable nonetheless because they areweakly complete for E (i.e., they are in E and hard for more than a measure 0 subset of E). The main result of this paper shows that Lutz's intuition is at least partially correct: many more problems are weakly complete for E than are complete for E.The main result of this paper states that weakly complete problems arenot rare in the sense that they form a non-measure 0 subset of E. This extends a recent result of Lutz that establishes the existence of problems that are weakly complete, but not complete, for E. The proof of Lutz's original result employs a sophisticatedmartingale diagonalization argument. Here, we simplify and extend Lutz's argument to prove the main result. This simplified martingale diagonalization argument may be applicable to other questions involving individual weakly complete problems.     

Invariance properties of RAMS and linear time

An operation on integers isLTTC if it is computable in linear time on a Turing machine (using the dyadic or binary representation of integers). AnLTTC-RAM (respectivelyI-RAM) is a RAM which only uses LTTC operations (respectively operations in the setI).The address-free time complexity measure of a RAM evaluates execution times using the logarithmic cost criterion but assumes that addressing operations are performed for free.     

Polynomial Approximation and Graph-Coloring

Consider a graph G=(V,E) of order n. In the minimum graph-coloring problem we try to color V with as few colors as possible so that no two adjacent vertices receive the same color. This problem is among the first ones proved to be intractable, and hence, it is very unlikely that an optimal polynomial-time algorithm could ever be devised for it. In this paper, we survey the main polynomial time approximation algorithms (the ones for which theoretical approximability bounds have been studied) for the minimum graph-coloring and we discuss their approximation performance and their complexity. Finally, we further improve the approximation ratio for graph-coloring. Received October 5, 2001; revised November 15, 2002 Published online: February 20, 2003     

Space-efficient recognition of sparse self-reducible languages

Mahaney and others have shown that sparse self-reducible sets have time-efficient algorithms, and have concluded that it is unlikely that NP has sparse complete sets. Mahaney's work, intuition, and a 1978 conjecture of Hartmanis notwithstanding, nothing has been known about the density of complete sets for feasible classes until now. This paper shows that sparse self-reducible sets have space-efficient algorithms, and in many cases, even have time-space-efficient algorithms. We conclude that NL, NC ^k , AC ^k , LOG(DCFL), LOG(CFL), and P lack complete (or even Turing-hard) sets of low density unless implausible complexity class inclusions hold. In particular, if NL (respectively P, _k , or NP) has a polylog-sparse logspace-hard set, then NLSC (respectively PSC, _k , or PHSC), and if P has subpolynomially sparse logspace-hard sets, then PPSPACE.     

On the k-path cover problem for cacti

In this paper we investigate the k-path cover problem for graphs, which is to find the minimum number of vertex disjoint k-paths that cover all the vertices of a graph. The k-path cover problem for general graphs is NP-complete. Though notable applications of this problem to database design, network, VLSI design, ring protocols, and code optimization, efficient algorithms are known for only few special classes of graphs. In order to solve this problem for cacti, i.e., graphs where no edge lies on more than one cycle, we introduce the so-called Steiner version of the k-path cover problem, and develop an efficient algorithm for the Steiner k-path cover problem for cacti, which finds an optimal k-path cover for a given cactus in polynomial time.     

On the completeness of incidence calculus

Incidence calculus is a mechanism for uncertain reasoning originally introduced by Bundy. He suggested a set of inference rules for deriving new incidence bounds from a given set of lower and upper bounds of some propositions. However, it is important to demonstrate that the inference axioms are complete in any axiomatization. It is proved in this paper that inference rules used by Bundy are indeed complete.     

Makespan minimization in preemptive two machine job shops

In this note we investigate the NP-complete problem of minimizing the makespan in a preemptive two machine job shop. We present a polynomial time approximation algorithm with worst case ratio 3/2 for this problem, and we also argue that this is the best possible result that can be derived via our line of approach.     

Anamorphic 3D geometry

An anamorphic image appears distorted from all but a few viewpoints. They have been studied by artists and architects since the early fifteenth century. Computer graphics opens the door to anamorphic 3D geometry. We are not bound by physical reality nor a static canvas. Here we describe a simple method for achieving anamorphoses of 3D objects by utilizing a variation of a simple projective map that is well-known in the computer graphics literature. The novelty of this work is the creation of anamorphic 3D digital models, resulting in a tool for artists and architects.     

New compacton soliton solutions and solitary patterns solutions of nonlinearly dispersive Boussinesq equations

The special exact solutions of nonlinearly dispersive Boussinesq equations (called B(m,n) equations), u_tt−u_xx−a(uⁿ)_xx+b(u^m)_xxxx=0, is investigated by using four direct ansatze. As a result, abundant new compactons: solitons with the absence of infinite wings, solitary patterns solutions having infinite slopes or cups, solitary waves and singular periodic wave solutions of these two equations are obtained. The variant is extended to include linear dispersion to support compactons and solitary patterns in the linearly dispersive Boussinesq equations with m=1. Moreover, another new compacton solution of the special case, B(2,2) equation, is also found.