We present the deterministic model of chemical reactions and show why this is an important class of equations both from the point of view of the qualitative theory and of applications. 

Next, we review results on the positivity of the solutions of the model, starting from the continuously rediscovered results by Volpert (1972). The components of the solutions are either strictly positive or zero for all positive times of their domain. Which is which---this can be decided using the concept of Volpert indexes.

 As an application, we show one of the algorithms to find minimal sets of species that ensure the positivity of all the species concentrations during the domain of solutions.

Perturbation theory is a practically and conceptually important tool of physics, especially in quantum mechanics. It represents an ultimate method in the quantum chemical description of many-electron systems like atoms or molecules, where the Rayleigh-Schrödinger formulation is most frequently applied. In spite of 100 years of research, the sufficient and necessary conditions for the convergence of this infinite series is yet unknown. Even worse, the perturbation series is quite often divergent in many important applications.

After a short review on perturbation theory, we discuss some possibilities to accelerate convergence as well as some methods of resumming divergent series by means of complex analysis techniques. The latter include analytic continuation and the numerical solution of the inverse boundary problem, where the boundary values are sought for a partial differential equation on the complex plane in the knowledge of the solution of that equation in certain domain.