## culaSyvex ifail and abstol

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**1**of**1**### culaSyvex ifail and abstol

Hi admin,

according to the cula reference, the culaSyvex(double or single) api get ifail and abstol 2 special parametres, the abstol value is recommended to 2x machine epsilon which for double is 2.2204e-016 and for single is 1.1921e-007 in my PC. the ifail parametre indicate the indice of eigenvalue , where there is a culaNoError returned or not, 0 means no error and the Eigenvector converge.

my question is i tried many matrix and never get 0 in the ifail. i tried abstol=0 or 2.2204e-016, each one never got 0 in ifail, so what does the values in the ifail mean exactely. need some explanations plz.

according to the cula reference, the culaSyvex(double or single) api get ifail and abstol 2 special parametres, the abstol value is recommended to 2x machine epsilon which for double is 2.2204e-016 and for single is 1.1921e-007 in my PC. the ifail parametre indicate the indice of eigenvalue , where there is a culaNoError returned or not, 0 means no error and the Eigenvector converge.

my question is i tried many matrix and never get 0 in the ifail. i tried abstol=0 or 2.2204e-016, each one never got 0 in ifail, so what does the values in the ifail mean exactely. need some explanations plz.

- asempey
**Posts:**11**Joined:**Tue Sep 21, 2010 1:52 am

### Re: culaSyvex ifail and abstol

IFAIL is an output INTEGER array of dimension N. That contains the index of the EIGENVECTORS that failed for converge. When returning a data error, the info will tell you how many eigenvectors failed to converge. So, for example, if the data error was 3, you should look at the first 3 elements of IFAIL to see their indexes.

This matches the behavior of the standard LAPACK implementation.

On another note, don't worry about the ABSTOL value. It's not related to the eigenvalues converging.

This matches the behavior of the standard LAPACK implementation.

On another note, don't worry about the ABSTOL value. It's not related to the eigenvalues converging.

- kyle
- Administrator
**Posts:**301**Joined:**Fri Jun 12, 2009 7:47 pm

### Re: culaSyvex ifail and abstol

kyle wrote:IFAIL is an output INTEGER array of dimension N. That contains the index of the EIGENVECTORS that failed for converge. When returning a data error, the info will tell you how many eigenvectors failed to converge. So, for example, if the data error was 3, you should look at the first 3 elements of IFAIL to see their indexes.

This matches the behavior of the standard LAPACK implementation.

On another note, don't worry about the ABSTOL value. It's not related to the eigenvalues converging.

thx for ur reply, appreciate it, however i give u an example of matrix [2 1;1 2] the result of culaDSyvex gives:

Eigenvalue: 1.000000 3.000000

ifail[0]= -532545

ifail[1]= -407961618

the value of ifail are both negative and they are surely not the index of eigenvector who cant converge. so maybe i got some wrong settings of parametre of culaDSyvex? but the Eigenvalue and eigenvector are right.

- asempey
**Posts:**11**Joined:**Tue Sep 21, 2010 1:52 am

### Re: culaSyvex ifail and abstol

The value of IFAIL when there is a no culaDataError is undetermined. Hence the gibberish.

- kyle
- Administrator
**Posts:**301**Joined:**Fri Jun 12, 2009 7:47 pm

### Re: culaSyvex ifail and abstol

Ok got it, thx for ur reply admin, i appreciate it.

- asempey
**Posts:**11**Joined:**Tue Sep 21, 2010 1:52 am

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