scholarly journals ON THE COMPLEXITY OF THE SUCCESSIVITY RELATION IN COMPUTABLE LINEAR ORDERINGS

2010 ◽  
Vol 10 (01n02) ◽  
pp. 83-99 ◽  
Author(s):  
ROD DOWNEY ◽  
STEFFEN LEMPP ◽  
GUOHUA WU
Keyword(s):  
New Method ◽  
Independent Interest ◽  
Linear Ordering ◽  
Turing Degrees ◽  
Linear Orderings ◽  
Open Question ◽  
Computably Enumerable

In this paper, we solve a long-standing open question (see, e.g. Downey [6, Sec. 7] and Downey and Moses [11]), about the spectrum of the successivity relation on a computable linear ordering. We show that if a computable linear ordering [Formula: see text] has infinitely many successivities, then the spectrum of the successivity relation is closed upwards in the computably enumerable Turing degrees. To do this, we use a new method of constructing [Formula: see text]-isomorphisms, which has already found other applications such as Downey, Kastermans and Lempp [9] and is of independent interest. It would seem to promise many further applications.

A Model of the Real Numbers

1963 ◽  
Vol 6 (2) ◽  
pp. 239-255
Author(s):  
Stanton M. Trott
Keyword(s):  
Irrational Number ◽  
Additive Group ◽  
Linear Ordering ◽  
Real Numbers ◽  
Ordered Group ◽  
The Real ◽  
Linear Orderings ◽  
Positive Elements ◽  
The Law ◽  
Ordered Pairs

The model of the real numbers described below was suggested by the fact that each irrational number ρ determines a linear ordering of J2, the additive group of ordered pairs of integers. To obtain the ordering, we define (m, n) ≤ (m', n') to mean that (m'- m)ρ ≤ n' - n. This order is invariant with group translations, and hence is called a "group linear ordering". It is completely determined by the set of its "positive" elements, in this case, by the set of integer pairs (m, n) such that (0, 0) ≤ (m, n), or, equivalently, mρ < n. The law of trichotomy for linear orderings dictates that only the zero of an ordered group can be both positive and negative.

A hierarchy for the plus cupping Turing degrees

2003 ◽  
Vol 68 (3) ◽  
pp. 972-988 ◽  
Author(s):  
Yong Wang ◽  
Angsheng Li
Keyword(s):  
Turing Degrees ◽  
Computably Enumerable

AbstractWe say that a computably enumerable (c. e.) degree a is plus-cupping, if for every c.e. degree x with 0 < x ≤ a, there is a c. e. degree y ≠ 0′ such that x ∨ y = 0′. We say that a is n-plus-cupping, if for every c. e. degree x, if 0 < x ≤ a, then there is a lown c. e. degree I such that x ∨ I = 0′. Let PC and PCn be the set of all plus-cupping, and n-plus-cupping c. e. degrees respectively. Then PC1 ⊆ PC2 ⊆ PC3 = PC. In this paper we show that PC1 ⊂ PC2, so giving a nontrivial hierarchy for the plus cupping degrees. The theorem also extends the result of Li, Wu and Zhang [14] showing that LC1 ⊂ LC2, as well as extending the Harrington plus-cupping theorem [8].

scholarly journals The role of the range of dispersal in a nonlocal Fisher-KPP equation: an asymptotic analysis

2020 ◽  
pp. 2050032
Author(s):  
Julien Brasseur
Keyword(s):  
Asymptotic Behavior ◽  
Asymptotic Analysis ◽  
Positive Solution ◽  
Nonlocal Problem ◽  
Type Equation ◽  
Independent Interest ◽  
Kpp Equation ◽  
Open Question

In this paper, we study the asymptotic behavior as [Formula: see text] of solutions [Formula: see text] to the nonlocal stationary Fisher-KPP type equation [Formula: see text] where [Formula: see text] and [Formula: see text]. Under rather mild assumptions and using very little technology, we prove that there exists one and only one positive solution [Formula: see text] and that [Formula: see text] as [Formula: see text] where [Formula: see text]. This generalizes the previously known results and answers an open question raised by Berestycki et al. Our method of proof is also of independent interest as it shows how to reduce this nonlocal problem to a local one. The sharpness of our assumptions is also briefly discussed.

LEARNING RECURSIVE LANGUAGES WITH BOUNDED MIND CHANGES

1993 ◽  
Vol 04 (02) ◽  
pp. 157-178 ◽  
Author(s):  
STEFFEN LANGE ◽  
THOMAS ZEUGMANN
Keyword(s):  
Language Learning ◽  
Learning Algorithms ◽  
New Method ◽  
Negative Data ◽  
Finite Sets ◽  
Exact Learning ◽  
Bounded Number ◽  
Positive Data ◽  
Open Question

In the present paper we study the learnability of enumerable families ℒ of uniformly recursive languages in dependence on the number of allowed mind changes, i.e. with respect to a well-studied measure of efficiency. We distinguish between exact learnability (ℒ has to be inferred w.r.t. ℒ) and class preserving learning (ℒ has to be inferred w.r.t. some suitable chosen enumeration of all the languages from ℒ) as well as between learning from positive and from both, positive and negative data. The measure of efficiency is applied to prove the superiority of class preserving learning algorithms over exact learning. In particular, we considerably improve results obtained previously and establish two infinite hierarchies. Furthermore, we separate exact and class preserving learning from positive data that avoids overgeneralization. Finally, language learning with a bounded number of mind changes is completely characterized in terms of recursively generable finite sets. These characterizations offer a new method to handle overgeneralizations and resolve an open question of Mukouchi.1

On Ehrenfeucht-Fraïssé equivalence of linear orderings

1990 ◽  
Vol 55 (1) ◽  
pp. 65-73 ◽  
Author(s):  
Juha Oikkonen
Keyword(s):  
Binary Relation ◽  
Linear Ordering ◽  
Relation Symbol ◽  
Linear Orderings ◽  
Binary Relation Symbol

AbstractC. Karp has shown that if α is an ordinal with ωα = α and A is a linear ordering with a smallest element, then α and α ⊗ A are equivalent in L∞ω up to quantifer rank α. This result can be expressed in terms of Ehrenfeucht-Fraïssé games where player ∀ has to make additional moves by choosing elements of a descending sequence in α. Our aim in this paper is to prove a similar result for Ehrenfeucht-Fraïssé games of length ω1. One implication of such a result will be that a certain infinite quantifier language cannot say that a linear ordering has no descending ω1-sequences (when the alphabet contains only one binary relation symbol). Connected work is done by Hyttinen and Oikkonen in [H] and [O].

scholarly journals Computably enumerable Turing degrees and the meet property

2015 ◽  
Vol 144 (4) ◽  
pp. 1735-1744 ◽  
Author(s):  
Benedict Durrant ◽  
Andy Lewis-Pye ◽  
Keng Meng Ng ◽  
James Riley
Keyword(s):  
Turing Degrees ◽  
Computably Enumerable

Embedding finite lattices into the ideals of computably enumerable turing degrees

2001 ◽  
Vol 66 (4) ◽  
pp. 1791-1802
Author(s):  
William C. Calhoun ◽  
Manuel Lerman
Keyword(s):  
Turing Degrees ◽  
Sufficient Condition ◽  
Necessary And Sufficient Condition ◽  
Finite Lattices ◽  
Necessary And Sufficient ◽  
Lattice Of Ideals ◽  
Computably Enumerable

Abstract.We show that the lattice L20 is not embeddable into the lattice of ideals of computably enumerable Turing degrees (ℐ), We define a structure called a pseudolattice that generalizes the notion of a lattice, and show that there is a Π2 necessary and sufficient condition for embedding a finite pseudolattice into ℐ.

Kueker's conjecture for stable theories

1989 ◽  
Vol 54 (1) ◽  
pp. 207-220 ◽  
Author(s):  
Ehud Hrushovski
Keyword(s):  
Independent Interest ◽  
Linear Ordering ◽  
Stable Case ◽  
Skolem Functions

AbstractKueker's conjecture is proved for stable theories, for theories that interpret a linear ordering, and for theories with Skolem functions. The proof of the stable case involves certain results on coordinatization that are of independent interest.

Uniformly defined descending sequences of degrees

1976 ◽  
Vol 41 (2) ◽  
pp. 363-367 ◽  
Author(s):  
Harvey Friedman
Keyword(s):  
Unique Solution ◽  
Set Theory ◽  
Limit Point ◽  
Infinite Sequence ◽  
Linear Ordering ◽  
Natural Numbers ◽  
Linear Orderings ◽  
Nonstandard Models ◽  
Related Paper ◽  
Models Of Set Theory

This paper answers some questions which naturally arise from the Spector-Gandy proof of their theorem that the π11 sets of natural numbers are precisely those which are defined by a Σ11 formula over the hyperarithmetic sets. Their proof used hierarchies on recursive linear orderings (H-sets) which are not well orderings. (In this respect they anticipated the study of nonstandard models of set theory.) The proof hinged on the following fact. Let e be a recursive linear ordering. Then e is a well ordering if and only if there is an H-set on e which is hyperarithmetic. It was implicit in their proof that there are recursive linear orderings which are not well orderings, on which there are H-sets. Further information on such nonstandard H-sets (often called pseudohierarchies) can be found in Harrison [4]. It is natural to ask: on which recursive linear orderings are there H-sets?In Friedman [1] it is shown that there exists a recursive linear ordering e that has no hyperarithmetic descending sequences such that no H-set can be placed on e. In [1] it is also shown that if e is a recursive linear ordering, every point of which has an immediate successor and either has finitely many predecessors or is finitely above a limit point (heretofore called adequate) such that an H-set can be placed on e, then e has no hyperarithmetic descending sequences. In a related paper, Friedman [2] shows that there is no infinite sequence xn of codes for ω-models of the arithmetic comprehension axiom scheme such that each xn+ 1 is a set in the ω-model coded by xn, and each xn+1 is the unique solution of P(xn, xn+1) for some fixed arithmetic P.

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