1. Albeverio, S., Fenstad, J.E., Høegh-Krohn, R. and Lindstrøm, T. (1986) Nonstandard Methods in Stochastic Analysis and Mathematical Physics. Academic Press, Orlando etc.
2. Berberian, S. K. (1962) Approximate proper vectors, Proc. AMS 13, pp. 111–114.
3. Bernstein, A.R., Robinson, A. (1966) Solution of an invariant subspace problem of K.T. Smith and P.R. Halmos, Pacific J. Math. 16, pp. 421–431.
4. Buoni, S., Harte, R. Wickstead, A.W. (1977) Upper and lower Fredholm spectra I, Proc. AMS 66, pp. 309–314.
5. Chadwick, J.J.M., and Wickstead, A.W. (1977), A quotient of ultrapowers of Banach spaces and semi-Fredholm operators, Bull. London Math. Soc. 9, pp. 321–325.