Alexander P. Bakulev, Dmitry Shirkov, Inevitability and Importance of Non-Perturbative Elements in Quantum Field Theory, Proceedings of the 6th Mathematical Physics Meeting, Sept. 14–23, 2010, Belgrade, Serbia, pp. 27–54, Belgrade Institute of Physics [arXiv:1102.2380, ISBN 978-86-82441-30-4, book webpage]