Efficient preconditioner updates for shifted linear systems

We present a technique for building effective and low cost preconditioners for sequences of shifted linear systems (A+ αI)xα = b, where A is symmetric positive definite and α > 0. This technique updates a preconditioner for A, available in the form of an LDLT factorization, by modifying only the nonzero entries of the L factor in such a way that the resulting preconditioner mimics the diagonal of the shifted matrix and reproduces its overall behavior. This approach is supported by a theoretical analysis as well as by numerical experiments, showing that it works efficiently for a broad range of values of α.