In this topic we considering nonlinear ODE:

$ \frac{dx}{dt}= (x^4) \cdot a_1 \cdot sin(\omega_1 \cdot t)-a_1 \cdot sin(\omega_1 \cdot t + \frac{\pi}{2})$ – Chini ODE

And system of nonlinears ODE:

$ \frac{dx}{dt}= (x^4+y^4) \cdot a_1 \cdot sin(\omega_1 \cdot t)-a_1 \cdot sin(\omega_1 \cdot t + \frac{\pi}{2})$

$ \frac{dy}{dt}= (x^4+y^4) \cdot a_2 \cdot sin(\omega_2 \cdot t)-a_2 \cdot sin(\omega_2 \cdot t + \frac{\pi}{2})$

Chini ODE’s NDSolve in Mathematica:

`pars = {a1 = 0.25, \[Omega]1 = 1} sol1 = NDSolve[{x'[t] == (x[t]^4) a1 Sin[\[Omega]1 t] - a1 Cos[\[Omega]1 t], x[0] == 1}, {x}, {t, 0, 200}] Plot[Evaluate[x[t] /. sol1], {t, 0, 200}, PlotRange -> Full] `

System of Chini ODE’s NDSolve in Mathematica:

`pars = {a1 = 0.25, \[Omega]1 = 3, a2 = 0.2, \[Omega]2 = 4} sol2 = NDSolve[{x'[t] == (x[t]^4 + y[t]^4) a1 Sin[\[Omega]1 t] - a1 Cos[\[Omega]1 t], y'[t] == (x[t]^4 + y[t]^4) a2 Sin[\[Omega]2 t] - a2 Cos[\[Omega]2 t], x[0] == 1, y[0] == -1}, {x, y}, {t, 0, 250}] Plot[Evaluate[{x[t], y[t]} /. sol2], {t, 0, 250}, PlotRange -> Full] `

There is no exact solution to these equations, therefore, the task is to obtain an approximate solution.

Using `AsymptoticDSolveValue`

was ineffective, because the solution is not expanded anywhere except point `0`

.

The numerical solution contains a strong periodic component; moreover, it is necessary to evaluate the oscillation parameters. Earlier, we solved this problem with some users as numerically: Estimation of parameters of limit cycles for systems of high-order differential equations (n> = 3)

*How to approximate the solution of the equation by the Fourier series so that it contains the parameters of the original differential equation in symbolic form, namely $ a_1$ , $ \omega_1$ , $ a_2$ and $ \omega_2$ .*