CFMultigrid

Helmholtz operator

The diag term is diag = C+D with

• D = 1/2(dx^2+dy^2+dz^2), the Poisson part
• C, the identity part

idiag is idiag = 1/diag, like for Poisson

During the coarsening, we keep D constant (it should be divided by 4). To compensate, C must multiplied by 4.

During the coarsening, restriction averages x then multiplies by

1. To set the diagonal on the coarsened grid the technique is simple:

apply the restriction on the coefficient. This is one in set_ope_coef