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Computability-theoretic and proof-theoretic aspects of partial and linear orderings. (English) Zbl 1044.03043
The authors examine the reverse mathematics and effective content of several variations of Szpilrajn’s Theorem [E. Szpilrajn, Fundam. Math. 16, 386–389 (1930; JFM 56.0843.02)]. A linear order type $$\tau$$ is called extendible if every partial order containing no subchains of type $$\tau$$ can be extended to a linear ordering containing no subchains of type $$\tau$$. The main results of the paper are: (1) “$$\omega ^*$$ is extendible” is provable in ACA$$_0$$ and strictly stronger than WKL$$_0$$; (2) “$$\eta$$ is extendible” is provable in $$\Pi^1_1$$-CA$$_0$$, but not in WKL$$_0$$; and (3) “$$\zeta$$ is extendible” is equivalent to ATR$$_0$$. Here $$\omega^*$$ is $$\omega$$ with the reverse order, $$\eta$$ is the order type of the rationals, and $$\zeta$$ is the order type of the integers.
For a survey of linear extensions of partial orderings, see R. Bonnet and M. Pouzet’s paper “Linear extensions of ordered sets” [in: I. Rival (ed.), Ordered sets, Proc. NATO Adv. Study Inst., Banff/Can. 1981, 125–170 (1982; Zbl 0499.06002)]. For background on computability-theoretic aspects of linear orderings, see R. G. Downey’s paper “Computability theory and linear orderings” [in: Yu. L. Ershov et al. (eds.), Handbook of recursive mathematics. Vol. 2. Recursive algebra, analysis and combinatorics. Amsterdam: Elsevier. Stud. Logic Found. Math. 139, 823–976 (1998; Zbl 0941.03045)].

##### MSC:
 03F35 Second- and higher-order arithmetic and fragments 03D45 Theory of numerations, effectively presented structures 03B30 Foundations of classical theories (including reverse mathematics) 06A05 Total orders 06A06 Partial orders, general 06A07 Combinatorics of partially ordered sets
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