Piecewise syndetic set

In mathematics, piecewise syndeticity is a notion of largeness of subsets of the natural numbers.

A set $S\subset \mathbb {N}$ is called piecewise syndetic if there exists a finite subset G of $\mathbb {N}$ such that for every finite subset F of $\mathbb {N}$ there exists an $x\in \mathbb {N}$ such that

$x+F\subset \bigcup _{n\in G}(S-n)$

where $S-n=\{m\in \mathbb {N} :m+n\in S\}$. Equivalently, S is piecewise syndetic if there is a constant b such that there are arbitrarily long intervals of $\mathbb {N}$ where the gaps in S are bounded by b.

Properties

• A set is piecewise syndetic if and only if it is the intersection of a syndetic set and a thick set.
• If S is piecewise syndetic then S contains arbitrarily long arithmetic progressions.
• A set S is piecewise syndetic if and only if there exists some ultrafilter U which contains S and U is in the smallest two-sided ideal of $\beta \mathbb {N}$, the Stone–Čech compactification of the natural numbers.
• Partition regularity: if $S$ is piecewise syndetic and $S=C_{1}\cup C_{2}\cup ...\cup C_{n}$, then for some $i\leq n$, $C_{i}$ contains a piecewise syndetic set. (Brown, 1968)
• If A and B are subsets of $\mathbb {N}$, and A and B have positive upper Banach density, then $A+B=\{a+b:a\in A,b\in B\}$ is piecewise syndetic

Other notions of largeness

There are many alternative definitions of largeness that also usefully distinguish subsets of natural numbers:

• Cofiniteness
• IP set
• member of a nonprincipal ultrafilter
• positive upper density
• syndetic set
• thick set

See also

• Ergodic Ramsey theory

References

• J. McLeod, "Some Notions of Size in Partial Semigroups" Topology Proceedings 25 (2000), 317-332
• Vitaly Bergelson, "Minimal Idempotents and Ergodic Ramsey Theory", Topics in Dynamics and Ergodic Theory 8-39, London Math. Soc. Lecture Note Series 310, Cambridge Univ. Press, Cambridge, (2003)
• Vitaly Bergelson, N. Hindman, "Partition regular structures contained in large sets are abundant", J. Comb. Theory (Series A) 93 (2001), 18-36
• T. Brown, "An interesting combinatorial method in the theory of locally finite semigroups", Pacific J. Math. 36, no. 2 (1971), 285–289.