# Does an existence of large cardinals have implications in number theory or combinatorics?

Does an existence of large cardinals have implications in more down-to-earth fields like number theory, finite combinatorics, graph theory, Ramsey theory or computability theory? Are there any interesting theorems in these areas that can be proved based on assumptions of existence of certain large cardinals? Or they are too far to change anything here?

I know that existence of certain cardinals implies consistency of some axiomatic systems of the set theory, which in turn can be expressed as a statement about certain huge Diophantine equations having no solutions, but it looks unlikely that we could use these Diophantine equations for anything else.

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Harvey Friedman has recently produced some results in this area. See for example Friedman, Invariant Maximal Cliques and Incompleteness, 2011. There is also a draft of a text book titled Boolean Relation Theory and Incompleteness at http://www.math.osu.edu/~friedman.8/manuscripts.html also by Harvey Friedman which is apparently also in this area.

In the paper it seems that Friedman has produced a graph theoretic theorem which is somewhat natural and requires a certain large cardinal axiom to prove.

Unfortunately I'm not particularly familiar with either of these works, so I'm not able to give a better explanation (although maybe another poster will be able to put a good explanation in their answer).

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My favorite example of Friedman's is one that I mentioned in my answer to a related MO question mathoverflow.net/questions/1924/… . It appears at the end of Martin Davis's Notices article: ams.org/notices/200604/fea-davis.pdf –  Timothy Chow May 4 '13 at 0:46
Regarding an "explanation," see Friedman's own succinct summary of the general methodology for coming up with these statements, which I reproduced here: mathoverflow.net/questions/27864/… –  Timothy Chow May 4 '13 at 0:52

There's an extremely elementary theorem whose only known proof relies on the existence of a rank-into-rank cardinal (basically the strongest large cardinal axiom not known to contradict ZFC).

Let $R_n = \mathbb{Z}/2^n\mathbb{Z}$.The $n$th Laver table is the unique binary operator $\star : R_n \times R_n \rightarrow R_n$ determined by the following conditions:

• $p \star 1 \equiv p + 1$
• $p \star (q \star r) \equiv (p \star q) \star (p \star r)$

Then the function $f_n(q) = 1 \star q$ is obviously periodic with some period $P(n)$ dividing $2^n$.

The existence of a rank-into-rank cardinal implies that $P(n)$ grows without bound.

Reference: Laver, Richard (1995), "On the algebra of elementary embeddings of a rank into itself"

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A recent short article: math.sciences.univ-nantes.fr/~lebed/Lebed_ATS14.pdf –  Todd Trimble Mar 27 at 15:09