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## Re: A * x = 0 for singular matrix

 From: Quentin Spencer Subject: Re: A * x = 0 for singular matrix Date: Fri, 23 Feb 2007 20:47:45 -0600 User-agent: Thunderbird 1.5.0.9 (Windows/20061207)

```lipschitz82 wrote:
```
By ~= 0 I mean approximately singular. It is singular up to my level of tolerance.
```
Thanks for the suggestion,
G.R.

```
On 2/23/07, *Quentin Spencer* <address@hidden <mailto:address@hidden>> wrote:
```
lipschitz82 wrote:
> I need to calculate the vector s.t. Ax = 0, for det(A) ~= 0. I've
> looked at the documentation, octave-forge, etc. but I couldn't
find a
> routine for doing this. Matrix A is pretty big, ie. in the
hundreds,
> but I also have a lot of computer time if the available routine
is slow.

Your subject line says A is singular, but your description of the
problem says det(A)~=0, which means it is not singular, so which
is it?
Assuming A is singular, the "null" function will compute a set of
basis
vectors for the null space of A.

Quentin

```
```
```
Well, if it's nearly but not exactly singular, then I would think the best solution is to compuate a singular value decomposition (see the svd function) and choose the singular vector corresponding to the smallest singular value.
```Quentin

```

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