INTERSECTIONS OF THE LEFT AND RIGHT ESSENTIAL SPECTRA OF $2\times 2$ UPPER TRIANGULAR OPERATOR MATRICES
YUAN LI, Xiu-Hong Sun, Hong-Ke Du · Bulletin of the London Mathematical Society · 2004
In this paper, perturbations of the left and right essential spectra of 2 × 2 upper triangular operator matrix MC are studied, where MC=(AC0B) is an operator acting on the Hilbert space H ⊕ K. For given operators A and B, the sets ∩C∈B(K,H)σle(MC) and ∩C∈B(K,H)σre(MC) are determined, where σle(T) and σre(T) denote, respectively, the left essential spectrum and the right essential spectrum of an operator T. 2000 Mathematics Subject Classification 47A10, 47A55.