Qualitative study of linear and nonlinear relaxation equations with $\psi$-Riemann-Liouville fractional derivatives

In the present paper, we consider the linear and nonlinear relaxation equation involving $\psi$-Riemann-Liouville fractional derivatives. By the generalized Laplace transform approach, the guarantee of the existence of solutions for the linear version is shown by Ulam-Hyer's stability. Then by establishing the method of lower and upper solutions along with Banach contraction mapping, we investigate the existence and uniqueness of iterative solutions for the nonlinear version with the non-monotone term. A new condition on the nonlinear term is formulated to ensure the equivalence between the solution of the nonlinear problem and the corresponding fixed point. Moreover, we discuss the maximal and minimal solutions to the nonlinear problem at hand. Finally, we provide two examples to illustrate the obtained results.