A representation of recursively enumerable sets through Horn formulas in higher recursion theory

Ever since Post [4] the structure of recursively enumerable sets and their classification has been an important area in recursion theory. It is also intimately connected with the study of the lattices and of r.e. sets and r.e. sets modulo finite sets respectively. (This lattice theoretic viewpoint was introduced by Myhill [3].) Key roles in both areas have been played by the lattice of r.e. supersets, , of an r.e. set A (along with the corresponding modulo finite sets) and more recently by the group of automorphisms of and . Thus for example we have Lachlan's deep result [1] that Post's notion of A being hyperhypersimple is equivalent to (or ) being a Boolean algebra. Indeed Lachlan even tells us which Boolean algebras appear as —precisely those with Σ3 representations. There are also many other simpler but still illuminating connections between the older typology of r.e. sets and their roles in the lattice . (r-maximal sets for example are just those with completely uncomplemented.) On the other hand, work on automorphisms by Martin and by Soare [8], [9] has shown that most other Post type conditions on r.e. sets such as hypersimplicity or creativeness which are not obviously lattice theoretic are in fact not invariant properties of .In general the program of analyzing and classifying r.e. sets has been directed at the simple sets. Thus the subtypes of simple sets studied abound — between ten and fifteen are mentioned in [5] and there are others — but there seems to be much less known about the nonsimple sets. The typologies introduced for the nonsimple sets begin with Post's notion of creativeness and add on a few variations. (See [5, §8.7] and the related exercises for some examples.) Although there is a classification scheme for r.e. sets along the simple to creative line (see [5, §8.7]) it is admitted to be somewhat artificial and arbitrary. Moreover there does not seem to have been much recent work on the nonsimple sets.

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Index sets in Ershov's hierarchy

Journal of Symbolic Logic ◽

10.2307/2272349 ◽

1974 ◽

Vol 39 (1) ◽

pp. 97-104 ◽

Cited By ~ 5

Author(s):

Jacques Grassin

Keyword(s):

Recursion Theory ◽

Case Note ◽

Boolean Combination ◽

Open Set ◽

Recursively Enumerable ◽

Power Set ◽

Index Sets ◽

Recursively Enumerable Sets ◽

Complete Set ◽

Recursive Isomorphism

This work is an attempt to characterize the index sets of classes of recursively enumerable sets which are expressible in terms of open sets in the Baire topology on the power set of the set N of natural numbers, usual in recursion theory. Let be a class of subsets of N and be the set of indices of recursively enumerable sets Wх belonging to .A well-known theorem of Rice and Myhill (cf. [5, p. 324, Rice-Shapiro Theorem]) states that is recursively enumerable if and only ifis a r.e. open set. In this case, note that if is not empty and does not contain all recursively enumerable sets, is a complete set. This theorem will be partially extended to classes which are boolean combinations of open sets by the following:(i) There is a canonical boolean combination which represents, namely the shortest among boolean combinations which represent.(ii) The recursive isomorphism type of depends on the length n of this canonical boolean combination (and trivial properties of ); for instance, is recursively isomorphic (in the particular case where is a boolean combination of recursive open sets) to an elementary set combination Yn or Un, constructed from {х ∣ х Wх) and depending on the length n. We can say also that is a complete set in the sense of Ershov's hierarchy [1] (in this particular case).

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The Sacks density theorem and Σ2-bounding

Journal of Symbolic Logic ◽

10.2307/2275670 ◽

1996 ◽

Vol 61 (2) ◽

pp. 450-467 ◽

Cited By ~ 7

Author(s):

Marcia J. Groszek ◽

Michael E. Mytilinaios ◽

Theodore A. Slaman

Keyword(s):

Recursion Theory ◽

Density Theorem ◽

Turing Degrees ◽

Recursively Enumerable ◽

Recursively Enumerable Sets ◽

Models Of Arithmetic

AbstractThe Sacks Density Theorem [7] states that the Turing degrees of the recursively enumerable sets are dense. We show that the Density Theorem holds in every model of P− + BΣ2. The proof has two components: a lemma that in any model of P− + BΣ2, if B is recursively enumerable and incomplete then IΣ1 holds relative to B and an adaptation of Shore's [9] blocking technique in α-recursion theory to models of arithmetic.

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An invariance notion in recursion theory

Journal of Symbolic Logic ◽

10.2307/2273381 ◽

1982 ◽

Vol 47 (1) ◽

pp. 48-66 ◽

Cited By ~ 3

Author(s):

Robert E. Byerly

Keyword(s):

Recursion Theory ◽

Recursive Functions ◽

Recursively Enumerable ◽

Recursively Enumerable Sets ◽

Partial Recursive Functions

AbstractA set of gödel numbers is invariant if it is closed under automorphisms of (ω, ·), where ω is the set of all gödel numbers of partial recursive functions and · is application (i.e., n · m ≃ φn(m)). The invariant arithmetic sets are investigated, and the invariant recursively enumerable sets and partial recursive functions are partially characterized.

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When catalytic P systems with one catalyst can be computationally complete

Journal of Membrane Computing ◽

10.1007/s41965-021-00079-x ◽

2021 ◽

Author(s):

Artiom Alhazov ◽

Rudolf Freund ◽

Sergiu Ivanov

Keyword(s):

Computational Complexity ◽

P Systems ◽

Membrane Systems ◽

Computational Completeness ◽

Recursively Enumerable ◽

Recursively Enumerable Sets

AbstractCatalytic P systems are among the first variants of membrane systems ever considered in this area. This variant of systems also features some prominent computational complexity questions, and in particular the problem of using only one catalyst in the whole system: is one catalyst enough to allow for generating all recursively enumerable sets of multisets? Several additional ingredients have been shown to be sufficient for obtaining computational completeness even with only one catalyst. In this paper, we show that one catalyst is sufficient for obtaining computational completeness if either catalytic rules have weak priority over non-catalytic rules or else instead of the standard maximally parallel derivation mode, we use the derivation mode maxobjects, i.e., we only take those multisets of rules which affect the maximal number of objects in the underlying configuration.

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α-degrees of α-theories

Journal of Symbolic Logic ◽

10.2307/2272413 ◽

1972 ◽

Vol 37 (4) ◽

pp. 677-682 ◽

Cited By ~ 1

Author(s):

George Metakides

Keyword(s):

Initial Segment ◽

Recursion Theory ◽

The Other ◽

Infinite Length ◽

Infinitary Logic ◽

Admissible Set ◽

Recursively Enumerable ◽

Limit Ordinal ◽

The Subject ◽

Further Development

Let α be a limit ordinal with the property that any “recursive” function whose domain is a proper initial segment of α has its range bounded by α. α is then called admissible (in a sense to be made precise later) and a recursion theory can be developed on it (α-recursion theory) by providing the generalized notions of α-recursively enumerable, α-recursive and α-finite. Takeuti [12] was the first to study recursive functions of ordinals, the subject owing its further development to Kripke [7], Platek [8], Kreisel [6], and Sacks [9].Infinitary logic on the other hand (i.e., the study of languages which allow expressions of infinite length) was quite extensively studied by Scott [11], Tarski, Kreisel, Karp [5] and others. Kreisel suggested in the late '50's that these languages (even which allows countable expressions but only finite quantification) were too large and that one should only allow expressions which are, in some generalized sense, finite. This made the application of generalized recursion theory to the logic of infinitary languages appear natural. In 1967 Barwise [1] was the first to present a complete formalization of the restriction of to an admissible fragment (A a countable admissible set) and to prove that completeness and compactness hold for it. [2] is an excellent reference for a detailed exposition of admissible languages.

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Interpretability over peano arithmetic

Journal of Symbolic Logic ◽

10.2307/2586787 ◽

1999 ◽

Vol 64 (4) ◽

pp. 1407-1425

Author(s):

Claes Strannegård

Keyword(s):

Modal Logic ◽

Completeness Theorem ◽

Peano Arithmetic ◽

Embedding Theorem ◽

Compactness Theorem ◽

Partial Answer ◽

Recursively Enumerable ◽

Recursively Enumerable Sets ◽

Interpretability Logic

AbstractWe investigate the modal logic of interpretability over Peano arithmetic. Our main result is a compactness theorem that extends the arithmetical completeness theorem for the interpretability logic ILMω. This extension concerns recursively enumerable sets of formulas of interpretability logic (rather than single formulas). As corollaries we obtain a uniform arithmetical completeness theorem for the interpretability logic ILM and a partial answer to a question of Orey from 1961. After some simplifications, we also obtain Shavrukov's embedding theorem for Magari algebras (a.k.a. diagonalizable algebras).

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