Monotonicity in the parameter of the Mittag-Leffler function and determining the fractional exponent of the subdiffusion equation

In this paper, we prove the strict monotonicity in the parameter $\rho$ of the Mittag-Leffler functions $E_{\rho }(-t^{\rho })$ and $t^{\rho -1}{E}_{\rho ,\rho }(-t^{\rho })$. Then, these results are applied to solve the inverse problem of determining the order of the fractional derivative in subdiffusion equations, where the available measurement is given at one point in space-time. In particular, we find the missing conditions in the previously known work in this area. Moreover, the obtained results are valid for a wider class of subdiffusion equations than those considered previously. An example of an initial boundary value problem constructed by Sh.A. Alimov is given, for which the inverse problem under consideration has a unique solution. We also point out the application of the monotonicity of the Mittag-Leffler functions to solving some other inverse problems of determining the order of a fractional derivative.