[petsc-users] Convergence of AMG

[Jed Brown]

Manav Bhatia <bhatiamanav at gmail.com> writes:

> Thanks, Jed. 
>
> The description says: “ Square the graph, ie. compute A'*A before aggregating it"
>
> What is A here? 

The original matrix, or its "graph" (your 6x6 blocks condensed to scalars).

> What is the impact of setting this to 0, which led to a very significant increase in the CPU time in my case? 

The aggregates are formed on the connectivity of your original matrix,
so root nodes are aggregated only with their first neighbors, resulting
in slower coarsening.