L. Giraud, A. Haidar and Y. Saad. (2010). Sparse Approximations of the Schur Complement for Parallel Algebraic Hybrid Solvers in 3<em>D</em>.
            <i>Numerical Mathematics: Theory, Methods and Applications</i>. <i>3</i> (3).
            276-294.
         doi:10.4208/nmtma.2010.33.2