On the steps of Emil Post : from normal systems to the correspondence decision problem

In 1946, Emil Leon Post (Bulletin of Amer. Math. Soc. 52 (1946), 264-268) introduced his famouscorrespondence decision problem, nowadays known as the Post Correspondence Problem (PCP).Post proved the undecidability of the PCP by areduction from his normal systems. In the presentarticle we follow the steps of Post, and give another, somewhat simpler and more straightforwardproof of the undecidability of the problem by using the same source of reductions as Post did.We investigate these, very different, techniques, and point out out some peculiarities in theapproach used by Post

Undecidability in Finite Transducers, Defense Systems and Finite Substitutions

In this manuscript we present a detailed proof for undecidability of the equivalence of finite substitutions on regular language b{0,1}*c. The proof is based on the works of Leonid P. Lisovik. </p

Undecidability in Finite Transducers, Defense Systems and Finite Substitutions

In this manuscript we present a detailed proof for undecidability of the equivalence of finite substitutions on regular language b{0,1}*c. The proof is based on the works of Leonid P. Lisovik. </p

Positivity of second order linear recurrent sequences

AbstractWe give a decision method for the Positivity Problem for second order recurrent sequences: it is decidable whether or not a recurrent sequence defined by un=aun-1+bun-2 has only nonnegative terms

Binary (generalized) Post Correspondence Problem

AbstractWe give a new proof for the decidability of the binary Post Correspondence Problem (PCP) originally proved in 1982 by Ehrenfeucht, Karhumäki and Rozenberg. Our proof is complete and somewhat shorter than the original proof although we use the same basic idea

Decision Problems on Copying and Shuffling

We study decision problems of the form: given a regular or linear context-free language $L$, is there a word of a given fixed form in $L$, where given fixed forms are based on word operations copy, marked copy, shuffle and their combinations

On the Steps of Emil Post: from Normal Systems to the Correspondence Decision Problem

In 1946 Emil Leon Post (Bull. Amer. Math. Soc. 52 (1946), 264-268) introduced his famous correspondence decision problem, nowadays known as the Post Correspondence Problem (PCP). Post proved the undecidability of the PCP by a reduction from his normal systems. In the present article we follow the steps of Post, and give another, somewhat simpler and more straightforward proof of the undecidability of the problem by using the same source of reductions as Post did. We investigate these, very different, techniques, and point out out some peculiarities in the approach taken by Post