An exact selection rule for l-forbidden M1 transitions in the particle-vibration coupling for degenerate single-particle oscillator shells

In the case of a degenerate single-particle oscillator shell, the particle-vibration coupling gives no contribution to M1 transitions which are l-forbidden in zeroth order; this is due to complete incoherence of all partial contributions. For a nondegenerate, realistic shell, l-forbiddeness is still approximately valid. (Auth.)