Three excitons states in nonlinear saturation -helix protein S. Issa, C. B. Tabi, H. P. Ekobena Fouda and T. C. Kofané Eur. Phys. J. Plus, 133 6 (2018) 233 Published online: 22 June 2018 DOI: 10.1140/epjp/i2018-12065-3