Äö¹ë Blockin Blockinðð Ìóòóú Êêêùððööþþøøóò Úúú

with a matrix A ∈ R, m ≥ n, of ill-determined rank. In parti ular, A is severely illonditioned and possibly singular. Matri es of this kind arise, for instan e, from the dis retization of ill-posed problems, su h as Fredholm integral equations of the rst kind; see, e.g., [10, 16℄ for dis ussions. The ve tor b ∈ R represents observations and is assumed to be ontaminated by an error e ∈ R, whi h may stem from measurement ina ura ies. Throughout this paper ‖ · ‖ denotes the Eu lidean ve tor norm or the asso iated indu ed matrix norm. Institute of Numeri al Simulation, Hamburg University of Te hnology, D-21071 Hamburg, Germany (joerg.lampe tu-harburg.de) Department of Mathemati al S ien es, Kent State University, Kent, OH 44242, USA (rei hel math.kent.edu) Institute of Numeri al Simulation, Hamburg University of Te hnology, D-21071 Hamburg, Germany (voss tu-harburg.de)