Bojan Tunguz tunguz [snail] g.a.l.com
Is there a way that a rank-four tensor can be brought to a
bi-diagonal form using some unitary group transformations? If
not an arbitrary four-tensor, is there a class of such tensors
that can be brought to this form?
Predrag:
what is 'bi-diagonal?' Diagonal in first index pair, and
diagonal in the second index pair? Not a chance, unless you
have some more specific properties, corresponding to
hermiticity or symmetry for a 2-index tensor (AKA matrix).