A five element basis for the uncountable linear orders. (English) Zbl 1143.03026

Summary: In this paper I show that it is relatively consistent with the usual axioms of mathematics (ZFC) together with a strong form of the axiom of infinity (the existence of a supercompact cardinal) that the class of uncountable linear orders has a five-element basis. In fact such a basis follows from the Proper Forcing Axiom, a strong form of the Baire Category Theorem. The elements are \(X\), \(\omega_1\), \(\omega_1^*\), \(C\), \(C^*\) where \(X\) is any suborder of the reals of cardinality \(\aleph_1\) and \(C\) is any Countryman line. This confirms a long-standing conjecture of Shelah.


03E35 Consistency and independence results
03E05 Other combinatorial set theory
03E40 Other aspects of forcing and Boolean-valued models
03E55 Large cardinals
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