The ultrapower T* of an arbitrary ordered set T is introduced as an infinitesimal extension of T. It is obtained as the set of equivalence classes of the sequences in T, where the corresponding relation is generated by a free ultrafilter on the set of natural numbers. It is established that T* always satisfies Cantor’s property, while one can give the necessary and sufficient conditions for T so that T* would be complete or it would fulfill the open completeness property, respectively. Namely, the density of the original set determines the open completeness of the extension, while independently, the completeness of T* is determined by the cardinality of T.
Boros, Z., and Száz, A. Infimum and supremum completeness properties of ordered sets without axioms. An. Ştiinţ. Univ. “Ovidius” Constanţa Ser. Mat. 16, 2 (2008), 31–37.
Corazza, P. Revisiting the construction of the real line. International Mathematical Forum 11, 2 (2016), 71–94. .
Jech, T. Set Theory. Springer-Verlag, Berlin – Heidelberg, 2003.
Łoś, J. On the extending of models i. Fundamenta Mathematicae 42 (1955), 38–54.
Luxemburg, W. A. What is nonstandard analysis? Amer. Math. Monthly 80, 6 (1973), 38–67.
Newman, D. J., and Parsons, T. D. On monotone subsequences. Amer. Math. Monthly 95, 1 (1988), 44–45.
Stroyan, K. D., and Luxemburg, W. A. J. Introduction to the theory of infinitesimals. Academic Press, New York – San Francisco – London, 1976.