On the strength of König's duality theorem for countable bipartite graphs
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AbstractLet CKDT be the assertion that for every countably infinite bipartite graph G, there exist a vertex covering C of G and a matching M in G such that C consists of exactly one vertex from each edge in M. (This is a theorem of Podewski and Stefifens [12].) Let ATR0 be the subsystem of second-order arithmetic with arithmetical transfinite recursion and restricted induction. Let RCA0 be the subsystem of second-order arithmetic with recursive comprehension and restricted induction. We show that CKDT is provable in ATR0. Combining this with a result of Aharoni, Magidor, and Shore [2], we see that CKDT is logically equivalent to the axioms of ATR0, the equivalence being provable in RCA0.
2017 ◽
Vol 10
(2)
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pp. 357-396
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1993 ◽
Vol 62
(1)
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pp. 51-64
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2010 ◽
Vol 16
(3)
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pp. 378-402
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