Two direct solvers for a system of linear equations
  A system of linear equations is normally understood as a linear mapping between two vector spaces. However, most direct solutions (e.g., QR, LU, ...) rely on the inelegant approach of back-substitution: a significant departure from such a characterization. In this paper, two new methods are introduced which respect the underlying vector space throughout, whether it be the row or column space of the coefficient matrix. Both solutions produce a generalized inverse as well as a projection operator for the null space. They also have the unique feature of admitting an online solution, which allows a solution to be computed as the data becomes available. 
Michael F. Zimmer
math.RA