1. S. Albeverio, J.E. Fenstad, R. Høegh-Krohn, T. Lindstrøm, Nonstandard Methods in Stochastic Analysis and Mathematical Physics (Academic Press, Orlando, 1986)
2. S. Albeverio, E. Gordon, A. Khrennikov, Finite dimensional approximations of operators in the spaces of functions on locally compact abelian groups. Acta Appl. Math. 64, 33–73 (2000)
3. H. Ando, U. Haagerup, Ultraproducts of von Neumann algebras. J. Funct. Anal. 266, 6842–6913 (2014)
4. S.K. Berberian, Approximate proper vectors. Proc. Am. Math. Soc. 13, 111–114 (1962)
5. S. Baratella, S.A. Ng, Some properties of nonstandard hulls of Banach algebras. Bull. Belg. Math. Soc. Simon Stevin 18, 31–38 (2011)