References
- Unbounded linear operators S.Goldberg
- Bollenttino U.M.I. v.11-B Ultrapowers and semi-Fredholm operators M.Gonzalez;A.marinez-Abejon
- Invertibility and singularity for bounded linear operators R.Harte
- J. Reine Angew. Math. v.313 Ultraproducts in Banach space theory S.Heinrich
- J. Korean Math. Soc. v.29 Almost regular operators Woo Young Lee;Chun In Choi