## Computing f(A)b via Least Squares Polynomial Approximations

Title | Computing f(A)b via Least Squares Polynomial Approximations |

Publication Type | Journal Article |

Year of Publication | 2009 |

Authors | Chen, J, Anitescu, M, Saad, Y |

Date Published | 11/2009 |

Other Numbers | ANL/MCS-P1693-1109 |

Abstract | Given a certain function f, various methods have been proposed in the past for addressing the important problem of computing the matrix-vector product f(A)b without explicitly computing the matrix f(A). Such methods were typically developed for a specific function f, a common case being that of the exponential. This paper discusses a procedure based on least squares polynomials that can, in principle, be applied to any (continuous) function f. The idea is to start by approximating the function by a spline of a desired accuracy. Then, a particular definition of the function inner product is invoked that facilitates the computation of the least squares polynomial to this spline function. Since the function is approximated by a polynomial, the matrix A is referenced only through a matrix-vector multiplication. In addition, the choice of the inner product makes it possible to avoid numerical integration. As an important application, we consider the case when f(t) = √t and A is a sparse, symmetric positive-definite matrix, which arises in sampling from a Gaussian process distribution. The covariance matrix of the distribution is defined by using a covariance function that has a compact support, at a very large number of sites that are on a regular or irregular grid. We derive error bounds and show extensive numerical results to illustrate the effectiveness of the proposed technique. |

http://www.mcs.anl.gov/papers/P1693.pdf |