Bruno Carpentieri Luc Giraud Serge Gratton Additive and Multiplicative Two-Level Spectral Preconditioning for General Linear Systems. 1593-1612 2007 29 SIAM J. Scientific Computing 4 http://dx.doi.org/10.1137/060654906 db/journals/siamsc/siamsc29.html#CarpentieriGG07