When A ∈ B(H) and B ∈ B(K) are given, we denote by M C an operator acting on the Hilbert space H ⊕ K of the form In this paper, first we give the necessary and sufficient condition for M C to be an upper semi-Fredholm (lower semi–Fredholm, or Fredholm) operator for some C ∈ B(K,H). In addition, let (A) ={λ ∈ ? : A − λI is not an upper semi-Fredholm operator} be the upper semi–Fredholm spectrum of A ∈ B(H) and let (A) = {λ ∈ ? : A − λI is not a lower semi–Fredholm operator} be the lower semi–Fredholm spectrum of A. We show that the passage from is accomplished by removing certain open subsets of from the former, that is, there is an equality
where is the union of certain of the holes in which happen to be subsets of Weyl's theorem and Browder's theorem are liable to fail for 2 × 2 operator matrices. In this paper, we also explore how Weyl's theorem, Browder's theorem, a–Weyl's theorem and a–Browder's theorem survive for 2 × 2 upper triangular operator matrices on the Hilbert space.