  This paper is a basis of a part of my set of lectures, subject:`Formal
methods in solving differential equations and constructing models of physical
phenomena'.
  Addressed, mainly: postgraduates and related readers.
  Content: a discussion of the simple models (I would rather say, toy models)
of the interaction based on equation arrays of the kind: d^2
u(t,x)/dt^2=c^{2}d^2 u(t,x)/dx^2 -4{\gamma_a}c\delta(x-x_a) Q_a(t)
-4{\gamma_b}c\delta(x-x_b) Q_b(t), Q_a(t) = u(t,x_a), Q_b(t) = u(t,x_b).
Central mathematical points: d'Alembert-Kirchhoff-like Formulae, Finite Rank
Perturbations.
