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