Using the example of a complicated problem such as the Cauchy problem for the Navier--Stokes equation, we show how the Poincare--Riemann--Hilbert boundary-value problem enables us to construct effective estimates of solutions for this case. The apparatus of the three-dimensional inverse problem of quantum scattering theory is developed for this. It is shown that the unitary scattering operator can be studied as a solution of the Poincare-Riemann--Hilbert boundary-value problem. The same scheme of reduction of Riemann integral equations for the zeta function to the Poincare--Riemann--Hilbert boundary-value problem allows us to construct effective estimates that describe the behaviour of the zeros of the zeta function very well.