I tried to use DGEEV to obtain correct eigenvalues for a 3*3 matrix but the wrong values were returned. I am new to this forum and lapack so I looked through the forum and saw that sometimes balancing the matrix can lead the computation to work incorrectly so i used DGEEVX. I tried all of the options for BALANC systematically, and none of them seemed to return the correct eigenvalues. My c++ code is featured below:

#include <iostream>

#include <cstdlib>

using namespace std;

extern "C" void dgeevx_(char * balanc, char * jobvl, char * jobvr, char *sense, int * n, double * a, int * lda, double * wr, double * wi, double * vl, int * ldvl,double *vr, int *ldvr, int * ilo, int * ihi, double * scale, double * abnrm, double * rconde, double *rcondv, double * work, int * lwork, int * iwork, int * info);

int main()

{

char balanc = 'N';

char jobvl = 'v';

char jobvr = 'v';

char sense = 'B';

int n = 3;

int lda = 3;

int ldvl = 3;

int ldvr = 3;

double a[lda][n] = {{2,1,-1},{0,2,0},{0,1,1}};

double wr[n];

double wi[n];

double vl[ldvl][n];

double vr[ldvr][n];

int ilo;

int ihi;

double scale[n];

double abnrm;

double rconde[n];

double rcondv[n];

int lwork = n*(n+6);

double work[lwork];

int iwork[2*n-2];

int info;

dgeevx_(&balanc,&jobvl,&jobvr,&sense, &n, &a[lda][n], &lda, &wr[n], &wi[n], &vl[ldvl][n], &ldvl, &vr[ldvr][n], &ldvr, &ilo, &ihi, &scale[n], &abnrm, &rconde[n], &rcondv[n], &work[lwork], &lwork, &iwork[2*n-2], &info);

cout<< "the eigenvalues are ";

cout<< "real....";

cout<<'\n';

for (int a=0;a<n;a++)

{

cout <<wr[a] <<" ";

}

cout <<'\n';

cout <<"Imaginary...";

cout <<'\n';

for (int b=0; b<n; b++)

{cout <<wi[b] <<" ";

}

return 0;

}

Thank you for reading this! If this question is answered somewhere else you can refer me to that answer.

Upper triangularly,

H a r

0 r y

0 0 0 (Harry)

I'm using LAPACK 3.0 if it makes a difference in evaluating bugs.