Soliton equations related to the affine Kac-Moody algebra D 4 (1) V. S. Gerdjikov, D. M. Mladenov, A. A. Stefanov and S. K. Varbev Eur. Phys. J. Plus, 130 6 (2015) 106 Published online: 08 June 2015 DOI: 10.1140/epjp/i2015-15106-5