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On uniform relationships between combinatorial problems. (English) Zbl 06560459
Summary: The enterprise of comparing mathematical theorems according to their logical strength is an active area in mathematical logic, with one of the most common frameworks for doing so being reverse mathematics. In this setting, one investigates which theorems provably imply which others in a weak formal theory roughly corresponding to computable mathematics. Since the proofs of such implications take place in classical logic, they may in principle involve appeals to multiple applications of a particular theorem, or to non-uniform decisions about how to proceed in a given construction. In practice, however, if a theorem $$\mathsf {Q}$$ implies a theorem $$\mathsf {P}$$, it is usually because there is a direct uniform translation of the problems represented by $$\mathsf {P}$$ into the problems represented by $$\mathsf {Q}$$, in a precise sense formalized by Weihrauch reducibility.
We study this notion of uniform reducibility in the context of several natural combinatorial problems, and compare and contrast it with the traditional notion of implication in reverse mathematics. We show, for instance, that for all $$n$$, $$j$$, $$k \geq 1$$, if $$j < k$$, then Ramsey’s theorem for $$n$$-tuples and $$k$$ many colors is not uniformly, or Weihrauch, reducible to Ramsey’s theorem for $$n$$-tuples and $$j$$ many colors. The two theorems are classically equivalent, so our analysis gives a genuinely finer metric by which to gauge the relative strength of mathematical propositions.
We also study Weak König’s Lemma, the Thin Set Theorem, and the Rainbow Ramsey’s Theorem, along with a number of their variants investigated in the literature. Weihrauch reducibility turns out to be connected with sequential forms of mathematical principles, where one wishes to solve infinitely many instances of a particular problem simultaneously. We exploit this connection to uncover new points of difference between combinatorial problems previously thought to be more closely related.

##### MSC:
 03B30 Foundations of classical theories (including reverse mathematics) 05D10 Ramsey theory 05D40 Probabilistic methods in extremal combinatorics, including polynomial methods (combinatorial Nullstellensatz, etc.) 05D15 Transversal (matching) theory 03D32 Algorithmic randomness and dimension 03D80 Applications of computability and recursion theory 03F35 Second- and higher-order arithmetic and fragments
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