We investigate the following three questions: Let n ∈ ℕ. For which Hausdorff spaces X is it true that whenever Γ is an arbitrary (respectively finite-to-one, respectively injective) function from ℕⁿ to X, there must exist an infinite subset M of ℕ such that Γ[Mⁿ] is discrete? Of course, if n = 1 the answer to all three questions is "all of them". For n ≥ 2 the answers to the second and third questions are the same; in the case n = 2 that answer is "those for which there are only finitely many points which are the limit of injective sequences". The answers to the remaining instances involve the notion of n-Ramsey limit. We also show that the class of spaces satisfying these discreteness conclusions for all n includes the class of F-spaces. In particular, it includes the Stone-Čech compactification of any discrete space.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-fm187-2-2,
author = {Timothy J. Carlson and Neil Hindman and Dona Strauss},
title = {Discrete n-tuples in Hausdorff spaces},
journal = {Fundamenta Mathematicae},
volume = {185},
year = {2005},
pages = {111-126},
zbl = {1095.54002},
language = {en},
url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-fm187-2-2}
}
Timothy J. Carlson; Neil Hindman; Dona Strauss. Discrete n-tuples in Hausdorff spaces. Fundamenta Mathematicae, Tome 185 (2005) pp. 111-126. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-fm187-2-2/