Alfredo Buttari Pasqua D'Ambra Daniela di Serafino Salvatore Filippone 2LEV-D2P4: a package of high-performance preconditioners for scientific and engineering applications. 223-239 2007 18 Appl. Algebra Eng. Commun. Comput. 3 http://dx.doi.org/10.1007/s00200-007-0035-z db/journals/aaecc/aaecc18.html#ButtariDSF07