On well quasi orders of free monoids

Lucian Ilie, Arto Salomaa
<span title="">1998</span> <i title="Elsevier BV"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/elaf5sq7lfdxfdejhkqbtz6qoq" style="color: black;">Theoretical Computer Science</a> </i> &nbsp;
The paper investigates down-sets associated to well quasi orders. Of particular languagetheoretic interest is the quasi order u <"t' (resp. u <,qu) of u being a subword (resp. a Par&h subword) of u, as well as their inverses. We establish a number of results about the regularity and effective regularity of the down-sets, in particular, a general condition for the effective regularity of the down-set associated to an inverse well quasi order (Theorem 7.1). It is decidable whether or not an
more &raquo; ... ary regular language results as a down-set from an infinite or bi-infinite word, whereas the same problem is undecidable for context-free language. A quasi order being a well quasi order is connected to arbitrary languages being confluent. @ 1998-Elsevier Science B.V. All rights reserved
<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="https://doi.org/10.1016/s0304-3975(98)00036-x">doi:10.1016/s0304-3975(98)00036-x</a> <a target="_blank" rel="external noopener" href="https://fatcat.wiki/release/63lqzb6fdrenpnvxm37b2ic4wa">fatcat:63lqzb6fdrenpnvxm37b2ic4wa</a> </span>
<a target="_blank" rel="noopener" href="https://web.archive.org/web/20170928104850/http://publisher-connector.core.ac.uk/resourcesync/data/elsevier/pdf/647/aHR0cDovL2FwaS5lbHNldmllci5jb20vY29udGVudC9hcnRpY2xlL3BpaS9zMDMwNDM5NzU5ODAwMDM2eA%3D%3D.pdf" title="fulltext PDF download" data-goatcounter-click="serp-fulltext" data-goatcounter-title="serp-fulltext"> <button class="ui simple right pointing dropdown compact black labeled icon button serp-button"> <i class="icon ia-icon"></i> Web Archive [PDF] <div class="menu fulltext-thumbnail"> <img src="https://blobs.fatcat.wiki/thumbnail/pdf/87/ec/87ec7438eff683b7dcb91ada671edd1c42499f53.180px.jpg" alt="fulltext thumbnail" loading="lazy"> </div> </button> </a> <a target="_blank" rel="external noopener noreferrer" href="https://doi.org/10.1016/s0304-3975(98)00036-x"> <button class="ui left aligned compact blue labeled icon button serp-button"> <i class="external alternate icon"></i> elsevier.com </button> </a>