Effective Hamiltonian approach to doubly degenerate electronic states - I — Theory and applications to E ⊗ (b1+b2) and related Jahn-Teller systemsF. Michelot and M. ReyEur. Phys. J. D, 33 3 (2005) 357-386DOI: https://doi.org/10.1140/epjd/e2005-00066-0