1. Abramowitz, M., Stegun, I.: Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. Natl. Bureau Stand. Appl. Math. Ser., vol. 55. U.S. Government Printing Office, Washington, D.C. (1964)
2. Bizon, P., Tabor, Z.: Formation of singularities for equivariant 2+1-dimensional wave maps into the 2-sphere. Nonlinearity 14(5), 1041–1053 (2001)
3. Bizon, P., Ovchinnikov, Y.N., Sigal, I.M.: Collapse of an instanton. Nonlinearity 17(4), 1179–1191 (2004)
4. Cazenave, T., Shatah, J., Tahvildar-Zadeh, A.S.: Harmonic maps of the hyperbolic space and development of singularities in wave maps and Yang–Mills fields. Ann. Inst. Henri Poincaré, Phys. Théor. 68(3), 315–349 (1998)
5. Christodoulou, D., Tahvildar-Zadeh, A.S.: On the regularity of spherically symmetric wave maps. Commun. Pure Appl. Math. 46, 1041–1091 (1993)