I am trying to solving a poisson equation in structured grid with Geometric Multigrid method.   However, when coarsening the fine grid, I simply double the grid spacing at each successive level.  That means that the number of points on the finest grid must be a power of 2. This is an undesirable constraint when doing some numerical simulations.Is there any strategy to avoid this constraint (i.e., to use grids whose sizes are not powers of 2?Thanks.  
How to use grid sizes that are not powers of two in Geometric Multigrid
multigrid