1. H.-J. Mikeska, A.K. Kolezhuk, in Quantum Magnetism, edited by U. Schollwöck, J. Richter, D.J.J. Farnell, R.F. Bishop (Springer, Berlin, 2004), p. 1
2. J. Richter, J. Schulenberg, A. Honecker, in Quantum Magnetism, edited by U. Schollwöck, J. Richter, D.J.J. Farnell, R.F. Bishop (Springer, Berlin, 2004), p. 85
3. I. Bose, Curr. Sci. 88, 62 (2005)
4. H. Bethe, Z. Phys. 71, 205 (1931)
5. B. Sutherland, in Beautiful Models (World Scientific, Singapore)