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
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