Alternative analysis to perturbation theory in quantum mechanics - Dyson series in matrix form J. Martínez-Carranza, F. Soto-Eguibar and H. Moya-Cessa Eur. Phys. J. D, 66 1 (2012) 22 Published online: 31 January 2012 DOI: 10.1140/epjd/e2011-20654-5