[petsc-dev] A small bug

[Barry Smith]

   Adam,

     Good eye for detecting this; it was an error. 

     I have pushed a fix for 3.3 and dev but rather than changing the diagonal entry I have changed the rhs for those locations. The reason is that for the geometric multigrid to converge well it is better to scale the "zeroed-out rows" diagonal associated with Dirichlet boundary conditions with a value near the rest of the diagonal entries.

    Barry

On Jul 19, 2012, at 12:59 PM, Adam Rose wrote:

> To whom it may concern,
> 
> I believe that I've found a small error in some example code located in $PETSC_DIR/src/ksp/ksp/examples/tutorials/ex45.c, and I just wanted to bring it to your attention.  It's a Laplace equation equation example with Dirichlet boundary conditions u=1 on the square [0,1]x[0,1], but on running the program I noticed that the boundary condition wasn't consistent with the solution.  This seems due to line 119 which sets the diagonal values of our discretization matrix in the case of boundary points.  It currently reads "v[0] = 2.0*(HxHydHz + HxHzdHy + HyHzdHx);" which is the same as the diagonal away from the boundary, but a quick change to "v[0] = Hx*Hy*Hz;" produces the desired result.
> 
> Thanks for reading,
> Adam Rose (Intern, Tech-X Corporation)