The investigation of the electromagnetic field in a regular waveguide filled with a homogeneous substance reduces to the study of two independent boundary value problems for the Helmholtz equation. In the case of a waveguide filled with an inhomogeneous substance, a relationship arises between the modes of these two problems, which in numerical experiments can not always be fully taken into account. In this paper, we will show how to rewrite the Helmholtz equations in the “Hamiltonian form” to express this connection explicitly. In this case, the problem of finding monochromatic waves in a waveguide with arbitrary filling will be reduced to an infinite system of ordinary differential equations in a properly constructed Hilbert space. The results of numerical experiments on finding normal waves, realized in the computer algebra system Sage, are presented.

Difference schemes that approximate dynamic systems are considered discrete models of the same phenomena that are described by continuous dynamic systems. Difference schemes with t-symmetry and midpoint and trapezoid schemes are considered. It is shown that these schemes are dual to each other, and, from this fact, we derive theorems on the inheritance of quadratic integrals by these schemes (Cooper’s theorem and its dual theorem on the trapezoidal scheme). Using examples of nonlinear oscillators, it is shown that these schemes poses challenges for theoretical research and practical application due to the problem of extra roots: these schemes do not allow one to unambiguously determine the final values from the initial values and vice versa. Therefore, we consider difference schemes in which the transitions from layer to layer in time are carried out using birational transformations (Cremona transformations). Such schemes are called reversible. It is shown that reversible schemes with t-symmetry can be easily constructed for any dynamical system with a quadratic right-hand side. As an example of such a dynamic system, a top fixed at its center of gravity is considered in detail. In this case, the discrete theory repeats the continuous theory completely: (1) the points of the approximate solution lie on some elliptic curve, which at $\Delta t\to 0$ turns into an integral curve; (2) the difference scheme can be represented using quadrature; and (3) the approximate solution can be represented using an elliptic function of a discrete argument. The last section considers the general case. The integral curves are replaced with closures of the orbits of the corresponding Cremona transformation as sets in the projective space over $\mathbb{R}$. The problem of the dimension of this set is discussed.