[This is the second part of Bill Gasarch’s guest post on “polynomial” versions of van der Waerden’s theorem. — Luca]
This is a the second of two Guest Postings on the Polynomial VDW Theorem. This one is on the Multi-dimensional Poly VDW Theorem.
For all , for any -coloring there exists , , such that
What would a multidimensional version of VDW look like? What would a multidimensional version of Poly VDW look like? Before stating that, we give corollaries to both to provide the flavor.
Corollary of Two-dimensional VDW:
For all , for any -coloring , there exists a square with all corners the same color. (Formally there exists ,, such that
Think of this as
Corollary of Two-dimensional Poly VDW:
For all , for any -coloring , there exists a rectangle with all corners the same color where the side is the square of its length. (Formally there exists , , such that
Think of this as
In the full Multidimensional theorems the set will be replaced by an arbitrary finite set.
Let . Let be a finite set of points in (e.g., , , , for the square case in the corollary to Multidim VDW). For all , for any -coloring , there exists , , , such that
This was proven by Furstenberg and Weiss (Topological dynamics and combinatorial number theory, Journal d’Analyse Mathematique, Vol. 34, 61–85, 1978); however, it was later observed that it follows from the (ordinary) Hales-Jewitt Theorem.
Furstenberg and Katznelson also proved a density version of the Multidimensional VDW Theorem (An ergodic Szemeredi’s theorem for commuting transformations, Journal d’Analyse Mathematique, Vol. 34, 275–291, 1978). Roughly speaking, if is dense enough then there exists , , such that .
Consider the functions
One may consider replacing these functions with a more complicated function of and . This leads to the following theorem.
Multidimensional Poly VDW:
Let . Let . Let be a finite set of points in . Let For all , for any -coloring , there exists , , , , such that
This was first proven by Bergelson and Leibman (Polynomial extensions of van der Waerden’s and Szemeredi’s theorems, Journal of the American Math Society 1996, Vol. 9, 725–753.
They actually proved a density version.
There is an alternative proof by Bergelson and Liebman (Set-polynomials and Polynomial extension of the Hales-Jewett Theorem, Annals of Math, Vol. 150, 1999, 33-75.}
The proof is in two steps
- Prove the Poly Hales-Jewett Theorem (henceforth Poly HJ). (This proof is not elementary.)
- Prove Multi-dimensional Poly VDW Theorem from Poly HJ. (This part was elementary.)
There is a purely combinatorial proof of the Multi-dimensional Poly VDW Theorem, though it is not stated in the literature:
- Walters has a combinatorial proof of Poly HJ in the paper of his mentioned in my last post.
- As noted above, Bergelson and Leibman showed how to get from the Poly HJ theorem to the Multi-dimensional Poly VDW theorem.
Putting all this together there is an elementary proof of the Multi-dimensional Poly VDW theorem. There is a general version as well, similar to the Gen Poly VDW from
my last post, but I won’t state it here.
These types of theorems have been generalized in Polynomial Szemeredi theorems for countable modules over integral domains and finite fields by Bergelson, Leibman, and McCutcheon, Journal d’Analyse Mathematique, Vol 95, 243–296, 2005.
There has also been some work on restricting what or could be in the above theorems. We state the easiest of such theorems. It is derivable from the (ordinary) Hales-Jewitt theorem.
VDW’s with restricted:
Let . Let be the set of all sums of distinct elements of . For all , for any -coloring there exists , , , such that
More complicated versions of this for multidimensional polynomials were proven by Bergelson and McCutcheon (An Ergodic IP Polynomial Szemeredi Theorem by Bergelson and McCutcheon. Memoirs of the American Math Society, Vol 46, 2000.
- Consider the one-dimensional VDW theorem over the reals. Is there an easy analytic proof of this, or of some subcases of it?
- If we allow polynomials with constant term what happens. More concretely: For all finite sets , determine such that:
- For any -coloring there exists , , such that is monochromatic.
- There exists a -coloring such that for all , , is not monochromatic.
- Everything I’ve mentioned above except the Poly HJ theorem has a density analoge that is difficult to proof. Find easier proofs. This is not well defined since it depends on how you define `easier’. Combinatorial may be one definition, but those can be rough too.
I would like to thank Alexander Leibman for his help in preparing this post.