Abstract
The ground term reachability problem consists in determining whether a given variablefree term t can be transformed into a given variablefree term t' by the application of rules from a term rewriting system R. The joinability problem, on the other hand, consists in determining whether there exists a variablefree term t'' which is reachable both from t and from t'. Both problems have proven to be of fundamental importance for several subfields of computer science. Nevertheless, these problems are undecidable even when restricted to linear term rewriting systems. In this work, we approach reachability and joinability in linear term rewriting systems from the perspective of parameterized complexity theory, and show that these problems are fixed parameter tractable with respect to the depth of derivations. More precisely, we consider a notion of parallel rewriting, in which an unbounded number of rules can be applied simultaneously to a term as long as these rules do not interfere with each other. A term t_1 can reach a term t_2 in depth d if t_2 can be obtained from t_1 by the application of d parallel rewriting steps. Our main result states that for some function f(R,d), and for any linear term rewriting system R, one can determine in time f(R,d)*t_1*t_2 whether a ground term t_2 can be reached from a ground term t_1 in depth at most d by the application of rules from R. Additionally, one can determine in time f(R,d)^2*t_1*t_2 whether there exists a ground term u, such that u can be reached from both t_1 and t_2 in depth at most d. Our algorithms improve exponentially on exhaustive search, which terminates in time 2^{t_1*2^{O(d)}}*t_2, and can be applied with regard to any linear term rewriting system, irrespective of whether the rewriting system in question is terminating or confluent.
BibTeX  Entry
@InProceedings{deoliveiraoliveira:LIPIcs:2017:6925,
author = {Mateus de Oliveira Oliveira},
title = {{Ground Reachability and Joinability in Linear Term Rewriting Systems are Fixed Parameter Tractable with Respect to Depth}},
booktitle = {11th International Symposium on Parameterized and Exact Computation (IPEC 2016)},
pages = {25:125:12},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {9783959770231},
ISSN = {18688969},
year = {2017},
volume = {63},
editor = {Jiong Guo and Danny Hermelin},
publisher = {Schloss DagstuhlLeibnizZentrum fuer Informatik},
address = {Dagstuhl, Germany},
URL = {http://drops.dagstuhl.de/opus/volltexte/2017/6925},
URN = {urn:nbn:de:0030drops69256},
doi = {10.4230/LIPIcs.IPEC.2016.25},
annote = {Keywords: Linear Term Rewriting Systems, Ground Reachability, Ground Joinability, Fixed Parameter Tractability}
}
Keywords: 

Linear Term Rewriting Systems, Ground Reachability, Ground Joinability, Fixed Parameter Tractability 
Collection: 

11th International Symposium on Parameterized and Exact Computation (IPEC 2016) 
Issue Date: 

2017 
Date of publication: 

09.02.2017 