1. M. Abramowitz and I. A. Stegun, Handbook of Mathematical Functions, Dover, New York, 1972.
2. R. A. Adams and J. J. F. Fournier, Sobolev Spaces, second edition, Academic Press, 2003.
3. V. Adamyan, Non-negative perturbations of nonnegative self-adjoint operators, Meth. Funct. Anal. Top. 13 (2007), no. 2, 103–109.
4. M. S. Agranovich, Spectral problems for second-order strongly elliptic systems in smooth and non-smooth domains, Russ. Math. Surv. 57:5 (2003), 847–920.
5. N. I. Akhiezer and I. M. Glazman, Theory of Linear Operators in Hilbert Space, Volume II, Pitman, Boston, 1981.