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Combinatorial principles weaker than Ramsey's Theorem for pairs

  • Denis R. Hirschfeldt (a1) and Richard A. Shore (a2)


We investigate the complexity of various combinatorial theorems about linear and partial orders, from the points of view of computability theory and reverse mathematics. We focus in particular on the principles ADS (Ascending or Descending Sequence), which states that every infinite linear order has either an infinite descending sequence or an infinite ascending sequence, and CAC (Chain-AntiChain), which states that every infinite partial order has either an infinite chain or an infinite antichain. It is wellknown that Ramsey's Theorem for pairs () splits into a stable version () and a cohesive principle (COH). We show that the same is true of ADS and CAC, and that in their cases the stable versions are strictly weaker than the full ones (which is not known to be the case for and ). We also analyze the relationships between these principles and other systems and principles previously studied by reverse mathematics, such as WKL0, DNR, and BΣ2. We show, for instance, that WKL0 is incomparable with all of the systems we study. We also prove computability-theoretic and conservation results for them. Among these results are a strengthening of the fact, proved by Cholak, Jockusch, and Slaman, that COH is -conservative over the base system RCA0. We also prove that CAC does not imply DNR which, combined with a recent result of Hirschfeldt, Jockusch. Kjos-Hanssen, Lempp, and Slaman, shows that CAC does not imply (and so does not imply ). This answers a question of Cholak, Jockusch, and Slaman.

Our proofs suggest that the essential distinction between ADS and CAC on the one hand and on the other is that the colorings needed for our analysis are in some way transitive. We formalize this intuition as the notions of transitive and semitransitive colorings and show that the existence of homogeneous sets for such colorings is equivalent to ADS and CAC, respectively. We finish with several open questions.



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[1]Ambos-Spies, K., Kjos-Hanssen, B., Lempp, S., and Slaman, T. A., Comparing DNR and WWKL, this Journal, vol.69 (2004), pp. 1089–1104.
[2]Arslanov, M., Cooper, S. B., and Li, A., There is no low maximal d.c.e. degree—corrigendum, Mathematical Logic Quarterly, vol. 50 (2004), pp. 628–636.
[3]Avigad, J., Notes on -conservativity, ω-submodels, and the collection schema, Technical Report CMU-PHIL-125, Carnegie Mellon, 2002, (updated version available at
[4]Cholak, P. A., Jockusch, C. G. Jr., and Slaman, T. A., On the strength of Ramsey's Theorem for pairs, this Journal, vol. 66 (2001), pp. 1–55.
[5]Cooper, S. B., Jump equivalence of the hyperhyperimune sets, this Journal, vol. 37 (1972), pp. 598–600.
[6]Downey, R., Hirschfeldt, D. R., Lempp, S., and Solomon, R., A set with no infinite low subset in either it or its complement, this Journal, vol.66 (2001), pp. 1371–1381.
[7]Downey, R. G., Computability theory and linear orderings, Handbook of recursive mathematics (Ershov, , Goncharov, , Nerode, , and Remmel, , editors), Studies in Logic and the Foundations of Mathematics, vol. 138–139, Elsevier, Amsterdam, 1998, pp. 823–976.
[8]Friedman, H., Systems of second order arithmetic with restricted induction I (abstract), this Journal, vol. 41 (1976), pp. 557–558.
[9]Giusto, M. and Simpson, S. G., Located sets and reverse mathematics, this Journal, vol. 65 (2000), pp. 1451–1480.
[10]Goncharov, S. S and Nurtazin, A. T., Constructive models of complete decidable theories, Algebra and Logic, vol. 12 (1973), pp. 67–77.
[11]Hájek, P., Interpretability and fragments of arithmetic, Arithmetic, Proof Theory, and Computational Complexity (Prague, 1991), Oxford Logic Guides, vol. 23, Oxford University Press, New York, 1993, pp. 185–196.
[12]Hájekand, P.Pudlák, P., Metamathematics of first-order arithmetic, Perspectives in Mathematical Logic, Springer-Verlag, Berlin, 1998, second printing.
[13]Harizanov, V. S., Turing degrees of certain isomorphic images of computable relations., Annals of Pure and Applid Logic, vol. 93 (1998), pp. 103–113.
[14]Herrmann, E., Infinite chains and antichains in computable partial orderings, this Journal, vol. 66 (2001), pp. 923–934.
[15]Hirschfeldt, D.R., Jockusch, C. G. Jr., Kjos-Hanssen, B., Lempp, S., and Slaman, T.A., Some remarks on the proof-theoretic strength of some combinatorial principles, to appear in the proceedings of the Program on Computational Prospects of Infinity, Singapore 2005.
[16]Hirst, J., Combinatorics in subsystems of second order arithmetic, Ph.D. Dissertation, Pennsylvania State University, 1987.
[17]Jockusch, C.G. Jr., Ramsey's Theorem and recursion theory, this Journal, vol. 37 (1972), pp. 268–280.
[18]Jockusch, C.G. Jr. and Soare, R. I., classes and degrees of theories, Transactions of the American Mathematical Society, vol. 173 (1972), pp. 33–56.
[19]Jockusch, C.G. Jr. and Stephan, F., A cohesive set which is not high, Mathematical Logic Quarterly, vol. 39 (1993), pp. 515–530, (correction in Mathematical Logic Quarterly vol. 43 (1997), p. 569).
[20]Lerman, M., On recursive linear orderings, Logic Year 1979–1980 (Lerman, , Schmerl, , and Soare, , editors), Lecture Notes in Mathematics, vol. 859, Springer-Verlag, Berlin, 1981, pp. 132–142.
[21]Mileti, J. R., Partition theorems and computability theory, Ph.D. Dissertation, University of Illinois at Urbana-Champaign, 2004.
[22]Mourad, J., Fragments of arithmetic and the foundations of the priority method, Ph.D. Dissertation, University of Chicago, 1988.
[23]Paris, J. B., A hierarchy of cuts in models of arithmetic, Model theory of algebra and arithmetic, Lecture Notes in Mathematics, vol. 834, Springer, Berlin-New York, 1980, pp. 312–337.
[24]Paris, J. B. and Kirby, L. A. S., Σn-collection schemas in arithmetic, Logic Colloquium '77, Studies in Logic and the Foundations of Mathematics, vol. 96, North-Holland, Amsterdam-New York, 1978, pp. 199–209.
[25]Rosenstein, J. G., Linear orderings, Pure and Applied Mathematics, vol. 98, Academic Press, New York etc, 1982.
[26]Seetapun, D. and Slaman, T. A., On the strength of Ramsey's Theorem, Notre Dame Journal of Formal Logic, vol. 36 (1995), pp. 570–582.
[27]Simpson, S. G., Degrees of unsolvability: a survey of results, Handbook of mathematical logic (Barwise, , editor), North-Holland, Amsterdam, 1977, pp. 631–652.
[28]Simpson, S. G., Subsystems of second order arithmetic, Perspectives in Mathematical Logic, Springer-Verlag, Berlin, 1999.
[29]Simpson, S. G. and Yu, X., Measure theory and weak König's Lemma, Archive for Mathematical Logic, vol. 30 (1990), pp. 171–180.
[30]Specker, E., Ramsey's Theorem does not hold in recursive set theory, Logic Colloquium '69, Studies in Logic and the Foundations of Mathematics, North-Holland, Amsterdam, 1971.
[31]Szpilrajn, E., Sur l'extension de l'ordre partiel, Fundamenta Mathematicae, vol. 16 (1930), pp. 386–389.


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