# sherman.morrison: Sherman-Morrison formula In fastmatrix: Fast Computation of some Matrices Useful in Statistics

## Description

The Sherman-Morrison formula gives a convenient expression for the inverse of the rank 1 update (\bold{A} + \bold{bd}^T) where \bold{A} is a n\times n matrix and \bold{b}, \bold{d} are n-dimensional vectors. Thus

(\bold{A} + \bold{bd}^T)^{-1} = \bold{A}^{-1} - \frac{\bold{A}^{-1}\bold{bd}^T \bold{A}^{-1}}{1 + \bold{d}^T\bold{A}^{-1}\bold{b}}.

## Usage

 1 sherman.morrison(a, b, d = b, inverted = FALSE) 

## Arguments

 a a numeric matrix. b a numeric vector. d a numeric vector. inverted logical. If TRUE, a is supposed to contain its inverse.

## Details

Method of sherman.morrison calls BLAS level 2 subroutines DGEMV and DGER for computational efficiency.

## Value

a square matrix of the same order as a.

## Examples

 1 2 3 4 5 n <- 10 ones <- rep(1, n) a <- 0.5 * diag(n) z <- sherman.morrison(a, ones, 0.5 * ones) z 

fastmatrix documentation built on Oct. 18, 2021, 5:08 p.m.