1. Åberg T, Jamison K A and Richard P 1976 Phys. Rev. Lett. 37:63
2. Åberg T 1977 Extended Abstracts of V Int. Conf. Vac. Ultrav. Rad. Phys., At. Mol. Phys., Montpellier, ed M C Castex, M Pouey and N Pouey, Vol I, p 73
3. Åberg T 1980 Phys. Scr. 21:495
4. Akhiezer A I and Berestetskii V B 1965 Quantum Electrodynamics Wiley, New York
5. Almbladh C O 1977 Phys. Rev. B 16:4343