I use ArnoldiSolver to compute eigenvalues for wave equation with periodic coefficients. Sometimes, it returns junk values that are clearly off (can be checked upon inspection) and unsorted. I use following statement :
vlam = ArnoldiSolver(a.mat, b.mat, fes.FreeDofs(), gf.vecs, -2)
It is weird that when the solver is ran again with the same input parameters, it pops out reasonable values. I would really appreciate your inputs on this problem.