Superperturbation solver for quantum impurity models H. Hafermann, C. Jung, S. Brener, M. I. Katsnelson, A. N. Rubtsov and A. I. Lichtenstein EPL, 85 2 (2009) 27007 Published online: 30 January 2009 DOI: 10.1209/0295-5075/85/27007