Numerical recipes

2.7 Sparse Linear Systems 77
Finally, solve the one further auxiliary problem
A · y = b (2.7.20)
In terms of these quantities, the solution is given by
�
x = y − Z · H · (V T �
· y)
(2.7.21)
Inversion by Partitioning
Once in a while, you will encounter a matrix (not even necessarily sparse)
that can be inverted efficiently by partitioning.
A is partitioned into
Suppose that the N × N matrix
�
P
A =
R
�
Q
S
(2.7.22)
where P and S are square matrices of size p × p and s × s respectively (p + s = N).
The matrices Q and R are not necessarily square, and have sizes p × s and s × p,
respectively.
If the inverse of A is partitioned in the same manner,
A −1 �
�P Q�
=
�R � �
(2.7.23)
S
then � P, � Q, � R, � S, which have the same sizes as P, Q, R, S, respectively, can be
found by either the formulas
�P =(P − Q · S −1 · R) −1
�Q = −(P − Q · S −1 · R) −1 · (Q · S −1 )
�R = −(S −1 · R) · (P − Q · S −1 · R) −1
�S = S −1 +(S −1 · R) · (P − Q · S −1 · R) −1 · (Q · S −1 )
or else by the equivalent formulas
�P = P −1 +(P −1 · Q) · (S − R · P −1 · Q) −1 · (R · P −1 )
�Q = −(P −1 · Q) · (S − R · P −1 · Q) −1
�R = −(S − R · P −1 · Q) −1 · (R · P −1 )
�S =(S − R · P −1 · Q) −1
(2.7.24)
(2.7.25)
The parentheses in equations (2.7.24) and (2.7.25) highlight repeated factors that
you may wish to compute only once. (Of course, by associativity, you can instead
do the matrix multiplications in any order you like.) The choice between using
equation (2.7.24) and (2.7.25) depends on whether you want � P or � S to have the
simpler formula; or on whether the repeated expression (S − R · P −1 · Q) −1 is easier
Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0-521-43108-5)
Copyright (C) 1988-1992 by Cambridge University Press. Programs Copyright (C) 1988-1992 by Numerical Recipes Software.
Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machinereadable
files (including this one) to any server computer, is strictly prohibited. To order Numerical Recipes books or CDROMs, visit website
http://www.nr.com or call 1-800-872-7423 (North America only), or send email to directcustserv@cambridge.org (outside North America).