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I'm trying to solve the following equation:

Sin[f t] == y[t] + y[t]^3 + y'[t] + (y'[t])^3

Where f is a constant, the frequency of the oscillation. I have tried using DSolve and NDSolve, but I cannot get a solution due to the cube in the equation. With NDSolve I gave initial conditions of

y[-Pi/f] == y[Pi/f]

and domain

{t, Pi/f, -Pi/f}

Does anybody know how this equation can possibly be solved?

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  • $\begingroup$ What is f? an expression or a parameter? $\endgroup$
    – zhk
    Apr 17, 2019 at 7:50
  • $\begingroup$ f is a constant. It's the frequency of the oscillation in my problem. Sorry. I edited the question. Thank you! $\endgroup$
    – Step
    Apr 17, 2019 at 7:51

1 Answer 1

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Use ParametricNDSolveValue

ode =  Sin[f Pi t] == \[Gamma][t] + \[Gamma][t]^3 +Derivative[1][\[Gamma]][t] + Derivative[1][\[Gamma]][t]^3
gamma = ParametricNDSolveValue[{ode, \[Gamma][0] == \[Gamma]0}, \[Gamma], {t, -10, 10}, {f, \[Gamma]0}]

to solve your problem for initial condition \[Gamma][0] == \[Gamma]0

What you call "intial condition" is only a restriction to force symmetrie \[Gamma][t]==\[Gamma][-t]

Perhaps you can calculate, for given f, the parameter \[Gamma]0 to restrict the symmetrie?

To force symmetrie at t=10 try ContourPlot

ContourPlot[gamma[f, \[Gamma]0][10] == gamma[f, \[Gamma]0][-10], {f, 1,3}, {\[Gamma]0, -.1, .1}, MaxRecursion -> 3,FrameLabel -> {f, \[Gamma]0}]

enter image description here

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  • $\begingroup$ Yes, I wanted to restrict the symmetry of the solution, since I am also expecting symmetry in Gamma (or y after editing). Can you expound on the calculating Gamma0? Apologies, I'm new to Mathematica and still familiarizing a lot of the syntax. $\endgroup$
    – Step
    Apr 17, 2019 at 8:08
  • $\begingroup$ What do you know about the parameters f,\[Gamma]0? $\endgroup$ Apr 17, 2019 at 8:21
  • $\begingroup$ f is a constant, it's the frequency I use in my experiments. The reason why I restrict the symmetry is because that's the only part of the curve that have values I know for sure, so the value at \[Gamma][0] and \[Gamma]0 itself are unknown to me. Does this make sense? $\endgroup$
    – Step
    Apr 17, 2019 at 8:24
  • $\begingroup$ @Step I edited my answer! $\endgroup$ Apr 17, 2019 at 9:15
  • $\begingroup$ I ran your suggestion on solving for the \[Gamma][0]==\[Gamma]0 but would you care to explain how the ContourPlot can force the symmetry at t=10? I think I have to keep the symmetry at the endpoints \[Gamma][-t]==\[Gamma][t] $\endgroup$
    – Step
    Apr 17, 2019 at 9:29

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