Journal Article sparse matrix Algorithms Chebyshev semi-iterative method Krylov subspace method LAPACK LSQR Lanczos process Tikhonov regularization conjugate-gradient method data encapsulation ill-posed problem iterative method linear equations linear least squares minimum-length solution minimum-residual method over determined system parallel computing preconditioning pseudoinverse solution random matrix random projection random sampling randomized algorithm rank-deficient regression ridge regression singular least-squares underdetermined system