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Covering of ordinals Braud, Laurent Ordinal theory pushdown hierarchy fundamental sequence monadic second-order logic prefix-recognizable graphs The paper focuses on the structure of fundamental sequences of ordinals smaller than $\e$. A first result is the construction of a monadic second-order formula identifying a given structure, whereas such a formula cannot exist for ordinals themselves. The structures are precisely classified in the pushdown hierarchy. Ordinals are also located in the hierarchy, and a direct presentation is given. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Laurent Braud 2009 Is Part Of LIPIcs, Volume 4, IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (2009) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/LIPIcs.FSTTCS.2009.2310 urn:nbn:de:0030-drops-23108 https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2009.2310 eng https://creativecommons.org/licenses/by-nc-nd/3.0/legalcode