An operational matrix method for nonlinear variable-order time fractional reaction–diffusion equation involving Mittag-Leffler kernelM. H. Heydari, A. Atangana, Z. Avazzadeh and M. R. MahmoudiEur. Phys. J. Plus, 135 2 (2020) 237DOI: https://doi.org/10.1140/epjp/s13360-020-00158-5