[1]  Wei Y,  Qiao S. The representation and approximation of the Drazin inverse if a linear operator in Hilbert
 space. Appl Math Comp,  2003, 138}(1): 77--89

[2]  Du H K, Deng C Y. The representation and characterization of Drazin inverses of operators on a Hilbert space. Linear  Algebra   Appl,  2005, 407}(1): 117--124

[3]  Barraa M, Boumazgour M.  A note on the spectra of upper triangular operator matrix.  Proc Amer Math Soc, 2003, 131(10): 3083--3088

[4]  Cao X H, Meng B. Essential appoximate point spectra and Weyl's theorem for operator matrices. J Math Anal Appl, 2005, 304(2): 759--771

[5]  Cao X H, et al. Weyl's theorem for upper triangular operator matrices. Linear Algebra  Appl, 2005, 420(1): 61--73

[6]  Djordjevi\'{c} D S. Perturbations spectra of operator matrices. J Operator Theroy, 2002, 48(2): 467--486

[7]  Djordjevi\'{c} D S, Stanimirovi\'{c} P S. On the generalized Drazin inverse and generalized resolvent. Czechoslovak Math J,  2001, 126(5): 617--634

[8]  Du H K,  Pan J. Perturbation of spectrums of 2×2 operator matrices.  Proc Amer Math Soc,  1994, 121(3): 761--776

[9]  Han J K, et al. Invertible completions of 2×2 operator matrices.  Proc Amer Math Soc, 1999, 128(1): 119--123

[10]  Lee W Y. Weyl's theorem  of operator matrices. Integr Equ Oper Theory,  1998, 32(1): 319--331

[11]  Lee W Y. Weyl spectra of operator matrices. Proc Amer Math Soc,  2000, 129(1): 131--138

[12]  Li Y,  et al. Intersections of the left and right essential spectra of 2×2 upper triangular operator matrices. Bull London Math Soc,  2004, 36: 811--819

[13]  Herrero D A.  Approximation of Hilbert Space Operators. Boston, London, Melbourne: Pitman Advanced Publishing Program, 1982