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Register a new userThe strength of Ramseys theorem for pairs and arbitrarily many colors
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In this paper, we show that $mathrm{RT}^{2}+mathsf{WKL}_0$ is a $Pi^{1}_{1}$-conservative extension of $mathrm{B}Sigma^0_3$.
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We study the reverse mathematics and computability-the-o-re-tic strength of (stable) Ramseys Theorem for pairs and the related principles COH and DNR. We show that SRT$^2_2$ implies DNR over RCA$_0$ but COH does not, and answer a question of Mileti by showing that every computable stable $2$-coloring of pairs has an incomplete $Delta^0_2$ infinite homogeneous set. We also give some extensions of the latter result, and relate it to potential approaches to showing that SRT$^2_2$ does not imply RT$^2_2$.
We prove that any proof of a $forall Sigma^0_2$ sentence in the theory $mathrm{WKL}_0 + mathrm{RT}^2_2$ can be translated into a proof in $mathrm{RCA}_0$ at the cost of a polynomial increase in size. In fact, the proof in $mathrm{RCA}_0$ can be found by a polynomial-time algorithm. On the other hand, $mathrm{RT}^2_2$ has non-elementary speedup over the weaker base theory $mathrm{RCA}^*_0$ for proofs of $Sigma_1$ sentences. We also show that for $n ge 0$, proofs of $Pi_{n+2}$ sentences in $mathrm{B}Sigma_{n+1}+exp$ can be translated into proofs in $mathrm{I}Sigma_{n} + exp$ at polynomial cost. Moreover, the $Pi_{n+2}$-conservativity of $mathrm{B}Sigma_{n+1} + exp$ over $mathrm{I}Sigma_{n} + exp$ can be proved in $mathrm{PV}$, a fragment of bounded arithmetic corresponding to polynomial-time computation. For $n ge 1$, this answers a question of Clote, Hajek, and Paris.
The Gratzer-Schmidt theorem of lattice theory states that each algebraic lattice is isomorphic to the congruence lattice of an algebra. We study the reverse mathematics of this theorem. We also show that the set of indices of computable lattices that are complete is $Pi^1_1$-complete; the set of indices of computable lattices that are algebraic is $Pi^1_1$-complete; the set of compact elements of a computable lattice is $Pi^{1}_{1}$ and can be $Pi^1_1$-complete; and the set of compact elements of a distributive computable lattice is $Pi^{0}_{3}$, and there is an algebraic distributive computable lattice such that the set of its compact elements is $Pi^0_3$-complete.
We analyze the strength of Hellys selection theorem HST, which is the most important compactness theorem on the space of functions of bounded variation. For this we utilize a new representation of this space intermediate between $L_1$ and the Sobolev space W1,1, compatible with the, so called, weak* topology. We obtain that HST is instance-wise equivalent to the Bolzano-Weierstrass principle over RCA0. With this HST is equivalent to ACA0 over RCA0. A similar classification is obtained in the Weihrauch lattice.
The well-known Galvin-Prikry Theorem states that Borel subsets of the Baire space are Ramsey: Given any Borel subset $mathcal{X}subseteq [omega]^{omega}$, where $[omega]^{omega}$ is endowed with the metric topology, each infinite subset $Xsubseteq omega$ contains an infinite subset $Ysubseteq X$ such that $[Y]^{omega}$ is either contained in $mathcal{X}$ or disjoint from $mathcal{X}$. Kechris, Pestov, and Todorcevic point out in their seminal 2005 paper the dearth of similar results for homogeneous structures. Such results are a necessary step to the larger goal of finding a correspondence between structures with infinite dimensional Ramsey properties and topological dynamics, extending their correspondence between the Ramsey property and extreme amenability. In this article, we prove an analogue of the Galvin-Prikry theorem for the Rado graph. Any such infinite dimensional Ramsey theorem is subject to constraints following from the 2006 work of Laflamme, Sauer, and Vuksanovic. The proof uses techniques developed for the authors work on the Ramsey theory of the Henson graphs as well as some new methods for fusion sequences, used to bypass the lack of a certain amalgamation property enjoyed by the Baire space.
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