A filter on is a consistent notion of largeness for subsets of . “Largeness” has the following properties. if is large and then is large if and are large then is large the empty set is not large At most one of and is large; an ultrafilter is a filter which always has an opinion …

# Category: Scrapbook

## Piecewise syndetic and van der Waerden

Joel Moreira has just proved that whenever the natural numbers are finitely coloured we can find and such that , and are all the same colour. He actually proves a much more general result via links to topological dynamics, but he includes a direct proof of this special case assuming only a consequence of van der Waerden’s …