[56] G. Theocharis, P.G. Kevrekidis, M. K. Oberthaler, and D. J. Frantzeskakis, Phys. Rev. A 76 , 045601 (2007). [57] F. Gerbier, Europhys. Lett. 66 , 771 (2004). [58] L. Salasnich, A. Parola, and L. Reatto, Phys. Rev. A 65 , 043614 (2002). [59] A. Mu˜ noz Mateo and V. Delgado, Phys. Rev. A 75 , 063610 (2007); ibid 77 , 013617 (2008); Annals of Phys. 324 , 709 (2009). [60] R. S. MacKay, in Stability of Equilibrium of Hamiltonian Systems , R. S. MacKay and J. D. Meiss (eds.) (Hilger, Bristol, 1987) p.137. [61] F. G. Bass, V. V. Konotop, and S. A. Puzenko, Phys. Rev. A 46 , 4185 (1992). [62] V. E. Zakharov and A. B. Shabat, Zh. Eksp. Teor. Fiz. 64 , 1627 (1973) [Sov. Phys. JETP 37 , 823 (1973)]. [63] K. J. Blow and N. J. Doran, Phys. Lett. A 107 , 55 (1985). [64] N. Akhmediev and A. Ankiewicz, Phys. Rev. A 47 , 3213 (1993). [65] L. Gagnon, J. Opt. Soc. Am. B 10 , 469 (1993). [66] Yu. S. Kivshar, T. J. Alexander, and S. K. Turitsyn, Phys. Lett. A 278 , 225 (2001). [67] P. G. Kevrekidis, V. V. Konotop, A. Rodrigues, and D. J. Frantzeskakis, J. Phys. B: At. Mol. Opt. Phys. 38 , 1173 (2005). [68] G. L. Alfimov and D. A. Zezyulin, Nonlinearity 20 , 2075 (2007). [69] Yu. S. Kivshar, and X. Yang, Phys. Rev. E 49 , 1657 (1994). [70] S. H. Strogatz, Nonlinear Dynamics and Chaos (Perseus Books, 2000). [71] C. Wang, G. Theocharis, P.G. Kevrekidis, N. Whitaker, K.J.H. Law, D.J. Frantzeskakis and B.A. Malomed, Phys. Rev. E 80 , 046611 (2009). 35