Eigenvectors of degenerate eigenvalues are not well-defined, and any linear combination is an acceptable solution. Interestingly, the results are not deterministic even when ARPACK is initialized with a deterministic v0. My guess is that, since the Krylov space spanned by v0 is just span(v0), ARPACK needs to enrich the subspace in some way, and it does so with random vectors.
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