Strictly hyperbolic systems of the first order differential equations, two-dimensional admissible areas boundary problems shift operator functional operator estimation of the spectral radius of the functional operator.

A Factorization Problem on a Smooth Two-Dimensional Surface
A. P. Soldatov A Factorization Problem on a Smooth Two-Dimensional Surface // Mathematical Notes, 2020, Vol. 108, No. 2, pp. 272–276. DOI: 10.1134/S0001434620070275