Modern methods for ODE with complex harmonic oscillations and phasors

Question

We have a differential equation of the following form:

$\frac{dx}{dt}=f(x) \cdot a \cdot \sin(\omega \cdot t)-a \cdot \sin(\omega \cdot t + \frac{\pi}{2})$

where $f(x)$ - arbitrary function from state variable, $a$ and $\omega$ - amplitude and frequency of harmonic.

The right side of this equation can be determined by harmonic oscillations addition, and the equation itself can be written in the following form:

$\frac{dx}{dt} =\sqrt{a^2+a^2 \cdot f(x)^2} \cdot \sin(\omega \cdot t + \phi(x))$ (1)

where $\phi(x) = \arcsin(\frac{a^2 \cdot f(x)^2}{\sqrt{a^2+a^2 \cdot f(x)^2}})$ - auxiliary phase

List of trigonometric identities - Linear Combinations

In addition, complex harmonic oscillation can be considered as a summation of phasors. Then, the differential equation takes the following form:

$\frac{dx}{dt} = f(x) \cdot a \cdot \cos(\omega \cdot t - \frac{\pi}{2})-a \cdot \cos(\omega \cdot t) = \mathrm{Re}(f(x) \cdot a e^{-i \cdot \frac{\pi}{2}} e^{i \cdot \omega \cdot t}) + \mathrm{Re}(-a e^{i \cdot 0} e^{i \cdot \omega \cdot t})$ (2)

Wiki - Phasor Addition

DSolve[x'[t] == x[t]^4 Sin[t] - Cos[t], x[t], t]

During evaluation of In[47]:= Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information.

Out[47]= DSolve[Derivative[1][x][t] == -Cos[t] + Sin[t] x[t]^4, x[t], t]

This equation is non-linear, and it’s unlikely that it can be solved “head-on” using DSolve. It is interesting to try to solve it using coordinate transformation, Lie symmetry groups, or the method of homotopy analysis.

1. Are these tools available in Mathematica?

2. Is it possible to obtain approximate solutions for qualitative and quantitative estimates of transient parameters. Expansion in rows is unacceptable, because due to the complexity of the solution, a large number of members of the series are required, and cutting off the excess makes it necessary to lose important information about the properties of the system.

Answer 1

Is it possible to somehow evaluate the qualitative and quantitative characteristics of such systems at least approximately?

One possible way is to use AsymptoticDSolveValue and obtain series solution approximation around different t values. For example, at t=0

AsymptoticDSolveValue[x'[t] == x[t]^4 Sin[t] - Cos[t], x[t], {t, 0, 4}]

gives

$$ \frac{1}{24} c_1{}^2 \left(36+12 c_1{}^5-c_1{}^2\right) t^4+\frac{1}{6} \left(1-8 c_1{}^3\right) t^3+\frac{c_1{}^4 t^2}{2}-t+c_1 $$

This is a Chini ode. Only special cases have known analytical solutions.

ode:=diff(x(t),t)=x(t)^4*sin(t)-cos(t);
DETools:-odeadvisor(ode)

           [_Chini]

More information about this type ode is at odeadvisor/Chini and a-nonlinear-first-order-ordinary-differential-equation

Maple can not solve it either. You could ask at math group, may be someone can come up with way to solve it.