Path: news.mathworks.com!not-for-mail From: "Steven Lord" <slord@mathworks.com> Newsgroups: comp.soft-sys.matlab Subject: Re: Inversion of a matrix Date: Thu, 18 Mar 2010 09:43:13 -0400 Organization: The MathWorks, Inc. Lines: 36 Message-ID: <hntala$qv6$1@fred.mathworks.com> References: <hnrlj3$nh8$1@fred.mathworks.com> Reply-To: "Steven Lord" <slord@mathworks.com> NNTP-Posting-Host: lords.dhcp.mathworks.com X-Trace: fred.mathworks.com 1268919786 27622 172.31.44.65 (18 Mar 2010 13:43:06 GMT) X-Complaints-To: news@mathworks.com NNTP-Posting-Date: Thu, 18 Mar 2010 13:43:06 +0000 (UTC) X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 6.00.2900.5512 X-RFC2646: Format=Flowed; Response X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Xref: news.mathworks.com comp.soft-sys.matlab:617892 "Matthew " <matthew.caulfield@ntlworld.com> wrote in message news:hnrlj3$nh8$1@fred.mathworks.com... > Hi there, I am trying to invert the following matrix (please copy and > paste into an ordinary text document to see the rows and columns). My > lecturer says it is possible using a tool which deals with sparse > matrices, but I can't find it anywhere. I have had a look at other posts > and there are some different methods for inverting matrices but none of > them seem to work. Three comments. 1) Don't invert a matrix unless you KNOW what you're doing and KNOW that you really, truly need the inverse. If you're inverting a matrix A so that you can solve A*x = b by using x = inv(A)*b, you do NOT need the inverse. Use the backslash operator instead: x = A\b. 2) As Roger has called out, column 8 of your matrix contains all zeros; therefore this matrix is singular and has no inverse. 3) Assuming that this matrix were invertible, if you were to invert your original version of A22 and the version you created by typing in the elements as displayed below, you may NOT receive the same results. By default, numbers in MATLAB are stored as double precision values -- that means roughly 16 digits of precision. As written below, we can't tell whether the first element of A22 is -0.0281000000000000, -0.0281499999999999, or something inbetween. That small difference may actually have a significant impact on the inverse of the matrix. -- Steve Lord slord@mathworks.com comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ