### Numerical problems in matrix factorization

Posted:

**Thu Feb 02, 2006 4:25 am**I am using MTL (a c++ interface to lapack), so I am not sure this is a Lapack or a MTL problem. In any case I ask this user group hoping in some hints.

Assume I try to solve the linear system Ax=b with

A=

[1,1,2 ],

[2,2,2],

[2,2,1]

and b:

[15],

[15],

[15]

obviously I get an error (A is singular and with this b there is no solution). In fact mtl method getsv, which interfaces to lapack, returns:

> Factorization failed with INFO = 2

However if I change A to

[1,1,2 ],

[2.1,2.1,2.1],

[2,2,1 ]

I get a solution!:

> A:

> 3x3

> [

> [1,1,2],

> [2.1,2.1,2.1],

> [2,2,1]

> ]

> b:

> 3x1

> [

> [15],

> [15],

> [15]

> ]

> x:

> 3x1

> [

> [-2.16004e+17],

> [2.16004e+17],

> [5]

> ]

> A:

> 3x3

> [

> [2.1,2.1,2.1],

> [0.952381,2.64545e-17,-1],

> [0.47619,0.5,1.5]

> ]

Thank you for any help,

fabio

This is clearly an error. Did anybody noticed it before?

Assume I try to solve the linear system Ax=b with

A=

[1,1,2 ],

[2,2,2],

[2,2,1]

and b:

[15],

[15],

[15]

obviously I get an error (A is singular and with this b there is no solution). In fact mtl method getsv, which interfaces to lapack, returns:

> Factorization failed with INFO = 2

However if I change A to

[1,1,2 ],

[2.1,2.1,2.1],

[2,2,1 ]

I get a solution!:

> A:

> 3x3

> [

> [1,1,2],

> [2.1,2.1,2.1],

> [2,2,1]

> ]

> b:

> 3x1

> [

> [15],

> [15],

> [15]

> ]

> x:

> 3x1

> [

> [-2.16004e+17],

> [2.16004e+17],

> [5]

> ]

> A:

> 3x3

> [

> [2.1,2.1,2.1],

> [0.952381,2.64545e-17,-1],

> [0.47619,0.5,1.5]

> ]

Thank you for any help,

fabio

This is clearly an error. Did anybody noticed it before?