1. Di Nasso, M.: Hypernatural numbers as ultrafilters. In: Loeb, P.A., Wolff, M. (eds.) Nonstandard Analysis for the Working mathematician, 2nd edn. Springer, Berlin (2015)
2. Di Nasso, M., Tachtsis, E.: Idempotent ultrafilters without Zorn’s Lemma. Proc. Am. Math. Soc. 146, 397–411 (2018)
3. Golan, G., Tsaban, B.: Hindman’s coloring theorem in arbitrary semigroups. J. Algebra 395, 111–120 (2013)
4. Hindman, N.: The existence of certain ultra-filters on
$${\mathbb{N}}$$
N
and a conjecture of Graham and Rothschild. Proc. Am. Math. Soc. 36, 341–346 (1972)