[SciPy-user] null space of a matrix

Ryan Krauss ryanfedora at comcast.net
Wed Jun 29 15:45:25 CDT 2005


Thanks to Nils and Robert for catching the complex conjugate thing.  I 
am going to be working with complex matrices and not knowing that may 
have caused me a bit of pain.

When you transpose a complex matrix in Matlab it always takes the 
complex conjugate transpose.  Is this desirable behavior for SciPy/Numeric?

Ryan

Nils Wagner wrote:

> On Wed, 29 Jun 2005 13:08:44 -0700
>  Robert Kern <rkern at ucsd.edu> wrote:
>
>> Ryan Krauss wrote:
>>
>>> This is a completely trivial addition to Richard's submission,  but 
>>> here
>>> is the function I am using to implement the null function if anyone 
>>> wants to copy and paste it into there own code:
>>>
>>> def null(A, eps=1e-15):
>>>    u, s, vh = scipy.linalg.svd(A)
>>>    null_mask = (s <= eps)
>>>    null_space = scipy.compress(null_mask, vh, axis=0)
>>>    return scipy.transpose(null_space)
>>>
>>> (this assumes you have already got an import scipy somewhere in the 
>>> file)
>>> (It made more sense to me to return column vectors.)
>>
>>
>> Point. You may also need to conjugate() that return value when you 
>> happen to be dealing with complex values; I'm not sure.
>
>
> Robert,
>
> Your exception is correct
>
> from scipy import *
> A = array(([1+0.1j,2,3],[1+0.1j,2,3],[1+0.1j,2+0j,3]))
> u, s, vh = linalg.svd(A)
> eps = 1.e-10
> null_mask = (s <= eps)
> null_space = compress(null_mask, vh, axis=0)
> print dot(A,transpose(conjugate(null_space)))
> print dot(A,transpose(null_space))
>
> Nils
>
>>
>> -- 
>> Robert Kern
>> rkern at ucsd.edu
>>
>> "In the fields of hell where the grass grows high
>>  Are the graves of dreams allowed to die."
>>   -- Richard Harter
>>
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>
>
>  
>
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