Laver's theorem
Laver's theorem, in order theory, states that order embeddability of countable total orders is a well-quasi-ordering. That is, for every infinite sequence of totally-ordered countable sets, there exists an order embedding from an earlier member of the sequence to a later member. This result was previously known as Fraïssé's conjecture, after Roland Fraïssé, who conjectured it in 1948;[1] Richard Laver proved the conjecture in 1971. More generally, Laver proved the same result for order embeddings of countable unions of scattered orders.[2][3] In reverse mathematics, the version of the theorem for countable orders is denoted FRA (for Fraïssé) and the version for countable unions of scattered orders is denoted LAV (for Laver).[4] In terms of the "big five" systems of second-order arithmetic, FRA is known to fall in strength somewhere between the strongest two systems, [math]\displaystyle{ \Pi_1^1 }[/math]-CA0 and ATR0, and to be weaker than [math]\displaystyle{ \Pi_1^1 }[/math]-CA0. However, it remains open whether it is equivalent to ATR0 or strictly between these two systems in strength.[5]
See also
References
- ↑ "Sur la comparaison des types d'ordres" (in French), Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences 226: 1330–1331, 1948, http://gallica.bnf.fr/ark:/12148/bpt6k31787/f1330; see Hypothesis I, p. 1331
- ↑ Harzheim, Egbert (2005), Ordered Sets, Advances in Mathematics, 7, Springer, Theorem 6.17, p. 201, doi:10.1007/b104891, ISBN 0-387-24219-8
- ↑ "On Fraïssé's order type conjecture", Annals of Mathematics 93 (1): 89–111, 1971, doi:10.2307/1970754
- ↑ Hirschfeldt, Denis R. (2014), Slicing the Truth, Lecture Notes Series of the Institute for Mathematical Sciences, National University of Singapore, 28, World Scientific; see Chapter 10
- ↑ Montalbán, Antonio (2017), "Fraïssé's conjecture in [math]\displaystyle{ \Pi_1^1 }[/math]-comprehension", Journal of Mathematical Logic 17 (2): 1750006, 12, doi:10.1142/S0219061317500064
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