Eigenvalue Solver Problems

Hi,

I try to solve an eigenvalue problem with the format " a = inner(nabla_grad(u), nabla_grad(v))*dx + u*v_ext*v*dx "

1. I don't know how to use other eigenvalue solvers except the default one
    What I know right now is to do as "eigensolver = SLEPcEigenSolver(A); eigensolver.solve()", but what eigenvalue solver is using here? how could we use other eigenvalue solvers?

2. When I set v_ext as 1/r (r is the distance from the point to the mesh center), it takes forever to get the results; and if I set v_ext as some other more common funtions (such as r, constant), it is pretty fast to solve.
I have changed the center a little bit to avoid the infinity, but it still takes very long, why?

thanks,
Houdong