[8] Berkani, M., Koliha, J. J.: 
Weyl type theorems for bounded linear operators. Acta Sci. Math. 69 (2003), 359-376. 
MR 1991673 | 
Zbl 1050.47014 
[10] Berkani, M., Zariouh, H.: 
Extended Weyl type theorems. Math. Bohem. 134 (2009), 369-378. 
MR 2597232 | 
Zbl 1211.47011 
[11] Berkani, M., Zariouh, H.: 
New extended Weyl type theorems. Mat. Vesn. 62 (2010), 145-154. 
MR 2639143 | 
Zbl 1258.47020 
[14] Djordjević, D. S.: 
Operators obeying $a$-Weyl's theorem. Publ. Math. 55 (1999), 283-298. 
MR 1721837 | 
Zbl 0938.47008 
[18] Rakočević, V.: 
Operators obeying $a$-Weyl's theorem. Rev. Roum. Math. Pures Appl. 34 (1989), 915-919. 
MR 1030982 
[19] Weyl, H.: 
Über beschränkte quadratische Formen, deren Differenz vollstetig ist. Rend. Circ. Mat. Palermo 27 German (1909), 373-392, 402. 
DOI 10.1007/BF03019655