Le Borne, SabineSabineLe Borne2021-10-262021-10-262008-01-25Linear Algebra and Its Applications 428 (11-12): 2455-2467 (2008-06-01)http://hdl.handle.net/11420/10617In this paper, we propose a new method to efficiently compute a representation of an orthogonal basis of the nullspace of a sparse matrix operator BT with B ∈ Rn × m, n > m. We assume that B has full rank, i.e., rank(B) = m. It is well-known that the last n - m columns of the orthogonal matrix Q in a QR factorization B = QR form such a desired null basis. The orthogonal matrix Q can be represented either explicitly as a matrix, or implicitly as a matrix H of Householder vectors. Typically, the matrix H represents the orthogonal factor much more compactly than Q. We will employ this observation to design an efficient block algorithm that computes a sparse representation of the nullspace basis in almost optimal complexity. This new algorithm may, e.g., be used to construct a null space basis of the discrete divergence operator in the finite element context, and we will provide numerical results for this particular application.en0024-3795200811-1224552467American Elsevier Publ.Block QR factorizationHierarchical matricesOrthogonal factorizationMathematikJournal Article10.1016/j.laa.2007.11.025Journal Article