# Error in linalg.inv ??

John Machin sjmachin at lexicon.net
Sat Jun 6 09:15:36 CEST 2009

```On Jun 6, 3:34 pm, Ajith Kumar <aj... at iuac.res.in> wrote:
> Hello,
>  I ran the following code (Using Debian 5.0)
>
> from numpy import *
> a = arange(1.,10.)
> b = reshape(a, [3,3])
> c = linalg.inv(b)
> print b
> print c
> print dot(b,c)
> print dot(c,b)
>
> And the result is
>
> [[ 1.  2.  3.]
>  [ 4.  5.  6.]
>  [ 7.  8.  9.]]
>
> [[  3.15221191e+15  -6.30442381e+15   3.15221191e+15]
>  [ -6.30442381e+15   1.26088476e+16  -6.30442381e+15]
>  [  3.15221191e+15  -6.30442381e+15   3.15221191e+15]]
>
> [[-0.5 -1.  -1. ]
>  [-1.  -2.   2. ]
>  [-1.5 -3.   1. ]]
>
> [[  5.5   8.   10.5]
>  [  3.    0.   -3. ]
>  [ -1.    0.   -3. ]]
>
> NOT the identity matrix. Any help ?

It's a longer time than I care to divulge since I took courses in
matrix algebra, but I do have a vague recollection that if determinant
(B) is zero, inverse(B) is not defined ... seeing the rows and columns
in B are linear (as are those of C), IIRC that means the determinants
are zero, and you are out of luck.

Are you ignoring exceptions? Is that _exactly_ what you typed in?
Try running it again, print the calculated determinants of B and C,
and tell what version of (a) numpy (b) Python you are using.

Isn't there a mailing list for numpy?

HTH,
John

```