Adomian decomposition method Riemann–Liouville fractional derivative existence existence and uniqueness fixed point fractional calculus fractional derivative fractional derivatives fractional differential equation fractional differential equations natural transform Aboodh transform iterative method Appell polynomials Atangana–Baleanu fractional derivative BAM neural networks BBM-Burger equation BHCS algorithm Babenko’s approach Banach fixed point theorem Banach space Bessel functions Bessel polynomials Boehmian CBDC COVID-19 Caputo derivative Caputo fractional derivative Caputo fractional derivatives Caputo q-derivative Caputo–Fabrizio and Atangana-Baleanu operators