[petsc-users] Least squares: using unpreconditioned LSQR

[Shri]

Use the option -pc_type none. 

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Hi, 

I wanted to solve some least squares problems using PETSc. My test matrix is size 3x2 but I wish to use this code for solving large ill-conditioned rectangular systems later. 

Looking at the PETSc manual I the found the KSPLSQR routine which implements the LSQR algorithm. 

However I am unsure how to use this routine. I am pasting the lines of the code which I use to set up the solver. 

Through the terminal I pass the option -ksp_type lsqr while running exectuable. 

ierr = KSPCreate(PETSC_COMM_WORLD,&ksp);CHKERRQ(ierr); 

ierr = KSPSetOperators(ksp,A,PETSC_NULL,DIFFERENT_NONZERO_PATTERN);CHKERRQ(ierr); 

ierr = KSPSetFromOptions(ksp);CHKERRQ(ierr); 

ierr = KSPSolve(ksp,b,x);CHKERRQ(ierr); 

ierr = KSPSolve(ksp,b,x);CHKERRQ(ierr); 

As you can see I have used PETSC_NULL for the preconditioner matrix since I wish to use the *unpreconditioned* version of the LSQR algorithm. This gives me an error. 

If I pass the matrix A it gives me an error again. I am not sure how to tell PETSc not to use a preconditioner. 

Could you pease tell me how I should use KSPSetOperators statement in this case to use the unpreconditioned algorithm. 

If you have a better sparse matrix least squares algorithm implemented please let me know. 




















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