the National Natural Science Foundation of China (10571099)
A∈B(H) is called Drazin invertible if A has finite ascent and descent. Let σD (A)={λ∈ C : A -λI is not Drazin invertible } be the Drazin .spectrum. This paper shows that if Mc =(A C 0 B)is a 2 × 2 upper t...