  In this work, an analogue of the Tricomi problem for equations of mixed type
with a fractional derivative is investigated. In one part of the domain, the
considered equation is a subdiffusion equation with a fractional derivative of
order ? 2 (0; 1) in the sense of Riemann-Liouville, and in the other it is a
wave equation. Assuming the parameter ? to be unknown, the corresponding
inverse problem is studied . It was found an additional condition, that
provides not only uniqueness but also existance of the desired parameter. It
should be noted that the inverse problem of determining the fractional
derivative for the subdiffusion and wave equations has been studied by many
mathematicians. But in the case of the Tricomi problem for a mixed-type
equation, the questions of determining the fractional time derivative are
studied for the first time.
