Continuum and undefine hole burning regions via pulse propagation in a four-level Doppler broadened atomic systemMuhammad Idrees, Humaira Kalsoom, Bakth Amin Bacha, Arif Ullah and Li-Gang WangEur. Phys. J. Plus, 135 9 (2020) 698DOI: https://doi.org/10.1140/epjp/s13360-020-00705-0