scholarly journals Monotone images of W-sets and hereditarily weakly confluent images of continua

2003 ◽  
Vol 74 (88) ◽  
pp. 111-114
Author(s):  
Jonathan Hatch ◽  
Caslav Stanojevic
Keyword(s):  
Surjective Function

A proper subcontinuum H of a continuum X is said to be a W-set provided for each continuous surjective function f from a continuum Y onto X, there exists a subcontinuum C of Y that maps entirely onto H. Hereditarily weakly confluent (HWC) mappings are those with the property that each restriction to a subcontinuum of the domain is weakly confluent. In this paper, we show that the monotone image of a W-set is a W-set and that there exists a continuum which is not in class W but which is the HWC image of a class W continuum.

scholarly journals Global Behavior of the Difference Equationxn+1=xn-1g(xn)

2014 ◽  
Vol 2014 ◽  
pp. 1-5
Author(s):  
Hongjian Xi ◽  
Taixiang Sun ◽  
Bin Qin ◽  
Hui Wu
Keyword(s):  
Continuous Function ◽  
Difference Equation ◽  
Positive Solution ◽  
Positive Solutions ◽  
Initial Conditions ◽  
Global Behavior ◽  
Initial Values ◽  
The Difference ◽  
Surjective Function

We consider the following difference equationxn+1=xn-1g(xn),n=0,1,…,where initial valuesx-1,x0∈[0,+∞)andg:[0,+∞)→(0,1]is a strictly decreasing continuous surjective function. We show the following. (1) Every positive solution of this equation converges toa,0,a,0,…,or0,a,0,a,…for somea∈[0,+∞). (2) Assumea∈(0,+∞). Then the set of initial conditions(x-1,x0)∈(0,+∞)×(0,+∞)such that the positive solutions of this equation converge toa,0,a,0,…,or0,a,0,a,…is a unique strictly increasing continuous function or an empty set.

Dual weak pigeonhole principle, pseudo-surjective functions, and provability of circuit lower bounds

2004 ◽  
Vol 69 (1) ◽  
pp. 265-286 ◽  
Author(s):  
Jan Krajíček
Keyword(s):  
Lower Bounds ◽  
Sparse Matrix ◽  
Proof System ◽  
Direct Construction ◽  
Proof Systems ◽  
Circuit Lower Bounds ◽  
Iterable Functions ◽  
Propositional Proof Systems ◽  
Propositional Proof ◽  
Surjective Function

AbstractThis article is a continuation of our search for tautologies that are hard even for strong propositional proof systems like EF. cf. [14, 15]. The particular tautologies we study, the τ-formulas. are obtained from any /poly map g; they express that a string is outside of the range of g, Maps g considered here are particular pseudorandom generators. The ultimate goal is to deduce the hardness of the τ-formulas for at least EF from some general, plausible computational hardness hypothesis.In this paper we introduce the notions of pseudo-surjective and iterable functions (related to free functions of [15]). These two properties imply the hardness of the τ-formulas from the function but unlike the hardness they are preserved under composition and iteration. We link the existence of maps with these two properties to the provability of circuit lower bounds, and we characterize maps g yielding hard τ-formulas in terms of a hitting set type property (all relative to a propositional proof system). We show that a proof system containing EF admits a pseudo-surjective function unless it simulates a proof system WF introduced by Jeřábek [11]. an extension of EF.We propose a concrete map g as a candidate function possibly pseudo-surjective or free for strong proof systems. The map is defined as a Nisan-Wigderson generator based on a random function and on a random sparse matrix. We prove that it is iterable in a particular way in resolution, yielding the output/input ratio n3 −ε (that improves upon a direct construction of Alekhnovich et al. [2]).

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