1. G. Misguich, C. Lhuillier, in Frustrated spin systems, edited by H.T. Diep (World-Scientific, 2005)
2. J. Richter, J. Schulenburg, A. Honecker, inQuantum Magnetism, edited by U. Schollwöck, J. Richter, D.J.J. Farnell, R.F. Bishop,Lecture Notes in Physics, 645 (Springer, Berlin, 2004), p. 85
3. M. Mambrini, A. Läuchli, D. Poilblanc, F. Mila,Phys. Rev. B 74, 144422 (2006)
4. J. Oitmaa, Zheng Weihong, Phys. Rev. B 54, 3022 (1996)
5. R.R.P. Singh, W. Zheng, J. Oitmaa, O.P. Sushkov, C.J. Hamer, Phys. Rev. Lett. 91, 017201 (2003)