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A model of parallel computation based on a generalization of nondeterminism in Turing machines is introduced. Complexity classes //T(n)-TIME, //L(n)-SPACE, //LOGSPACE, //PTIME, etc. are defined for these machines in a way analogous to T(n)-TIME, L(n)-SPACE, LOGSPACE, PTIME, etc. for deterministic machines. It is shown that, given appropriate honesty conditions, L(n)-SPACE ⊆ //L(n)2-TIME T(n)-TIME...
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In a wide variety of situations, computer science has found it convenient to define complex object as (fixed-point) solutions of certain equations. This has been done in both algebraic and order-theoretic settings, and has often been contrasted with other approaches. This paper shows how to formulate such solutions in a setting which encompasses both algebraic and order-theoretic aspects, so that...
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We define alternating Turing Machines which are like nondeterministic Turing Machines, except that existential and universal quantifiers alternate. Alternation links up time and space complexities rather well, in that alternating polynomial time equals deterministic polynomial space, and alternating linear space equals deterministic exponential time. Such considerations lead to a two-person game complete...
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This paper deals with logics of programs. The objective is to formalize a notion of program description, and to give both plausible (semantic) and effective (syntactic) criteria for the notion of truth of a description. A novel feature of this treatment is the development of the mathematics underlying Floyd-Hoare axiom systems independently of such systems. Other directions that such research might...
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We consider heuristics which attempt to maintain a binary search tree in a near optimal form, assuming that elements are requested with fixed, but unknown, independent probabilities. A "move to root" heuristic is shown to yield an expected search time within a constant factor of that of an optimal static binary search tree. On the other hand, a closely related "simple exchange"...
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To each family C of interpretations corresponds an equivalence relation among program schemes, namely the equivalence of the program schemes for all interpretation of C. A family C is algebraic if any two programs are C-equivalent iff every partial finite computation of one of them is C-equivalent to some partial finite computation of the other. Our main theorem states that a family C is algebraic...
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This paper presents a formulation, within the framework of initial algebra semantics, of Knuthian semantic systems (K-systems) which contain both synthesized and inherited attributes. This formulation permits a precise definition of K-systems, and combines their intuitive appeal with the theoretical power of algebraic methods. The basic approach consists of algebraically specifying the semantic portion...
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In this paper we investigate the structure of sets which are complete for various classes. We show, for example, that sets complete for deterministic time classes contain infinite polynomial time recognizable subsets, thus showing that they are not complex almost everywhere. We show by a related technique that any set complete for NEXP_TIME contains an infinite subset in DEXP_TIME, thereby showing...
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Program structure is defined in terms of a simple graph grammar, the "semi-structured flow graph grammar," which admits many of the control structure extensions suggested for "structured programming." The grammar defines a set of graph reductions which are shown to have the "Finite Church-Rosser (FCR)" property; i.e., when applied in any order to a graph, the limit (when...
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We develop optimal algorithms for forming the intersection of geometric objects in the plane and apply them to such diverse problems as linear programming, hidden-line elimination, and wire layout. Given N line segments in the plane, finding all intersecting pairs requires O(N2) time. We give an O(N log N) algorithm to determine whether any two intersect and use it to detect whether two simple plane...
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Let P1 = {w ε Σ*:w = wR, |w| ≫ 1} be the set of all nontrivial palindromes over Σ. In Part I, we present a linear-time on-line recognition algorithm for P1* ("palstar") on a random-access machine with addition and uniform cost criterion. We also present a lineartime on-line recognition algorithm for P12 on a multitape Turing machine and a recognition algorithm for P12 on a two-way deterministic...
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In an earlier paper [1], the author showed that certain problems involving sparse polynomials and integers are NP-hard. In this paper we show that many related problems are also NP-hard. In addition, we exhibit some new NP-complete problems. Most of the new results concern problems in which the nondeterminism is "hidden". That is, the problems are not explicitly stated in terms of one of...
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A basic question in the area of asynchronous computation is: Given a synchronization problem, what synchronization primitives are needed for a solution? This paper is directed toward answering this question by characterizing the "behavior" of synchronization systems incorporating PV, PV multiple, PV chunk and PV general synchronization primitives.
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Straight line programs in which array elements can be referenced and set are considered. Two programs are equivalent if they compute the same expression as a function of the inputs. Testing the equivalence of programs with arrays is shown to be NP-complete, while programs without arrays can be tested for equivalence in linear time. Equivalence testing takes polynomial time when programs have either...
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There are languages which can be recognized by a deterministic (k + 1)-headed oneway finite automaton but which cannot be recognized by a k-headed one-way (deterministic or non-deterministic) finite automaton. Furthermore, there is a language accepted by a 2-headed nondeterministic finite automaton which is accepted by no k-headed deterministic finite automaton.
