
A linear ordering is scattered if it does not contain a copy of the rationals. Hausdorff characterised the class of scattered linear orderings as the least family of linear orderings that includes the class of well-orderings and reversed well-orderings, and is closed under lexicographic sums with index set in . More generally, we say that a partial ordering is -scattered if it does not contain a copy of any -dense linear ordering. We prove analogues of Hausdorff's result for -scattered linear orderings, and for -scattered partial orderings satisfying the finite antichain condition. We also study the -scattered partial orderings, where is the saturated linear ordering of cardinality , and a partial ordering is -scattered when it embeds no copy of . We classify the -scattered partial orderings with the finite antichain condition relative to the -scattered linear orderings. We show that in general the property of being a -scattered linear ordering is not absolute, and argue that this makes a classification theorem for such orderings hard to achieve without extra set-theoretic assumptions.
Secondary 06A05 06A06 Scattered posets scattered chains classification well-quasi-orderings better-quasi-orderings finite antichain condition, better-quasi-orderings, finite antichain condition, 2010 Mathematics Subject Classification. Primary 06A07, 620, 510, Secondary 06A05, [MATH.MATH-LO]Mathematics [math]/Logic [math.LO], classification, well-quasi-orderings, FOS: Mathematics, 19999 Mathematical Sciences not elsewhere classified, 06A06 Scattered posets, scattered chains
Secondary 06A05 06A06 Scattered posets scattered chains classification well-quasi-orderings better-quasi-orderings finite antichain condition, better-quasi-orderings, finite antichain condition, 2010 Mathematics Subject Classification. Primary 06A07, 620, 510, Secondary 06A05, [MATH.MATH-LO]Mathematics [math]/Logic [math.LO], classification, well-quasi-orderings, FOS: Mathematics, 19999 Mathematical Sciences not elsewhere classified, 06A06 Scattered posets, scattered chains
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