1. Blass, A.: Ultrafilters related to Hindman’s finite-unions theorem and its extensions. In: Simpson, S. (ed.) Logic and combinatorics (Arcata, CA, 1985). Contemporary Mathematics, vol. 65, pp. 263–274. American Mathematical Society, Providence (1987)
2. Bretón, D.F.: Strongly summable ultrafilters, union ultrafilters, and the trivial sums property. Can. J. Math. (2015). doi: 10.4153/CJM-2015-023-9
3. Capraro, V., Lupini, M.: With an appendix by Vladimir Pestov. In: Introduction to Sofic and Hyperlinear Groups and Connes’ Embedding Conjecture. Lecture Notes in Mathematics, vol. 2136. Springer (2015)
4. Clifford, A.H., Preston, G.B.: The Algebraic Theory of Semigroups. Mathematical Surveys No. 7, vol. 2. American Mathematical Society, Providence (1967)
5. Fuchs, L.: Infinite Abelian Groups, vol. 1. Pure and Applied Mathematics, vol. 36. Academic Press, New York (1970)