# Solving Nonlinear ode in mathematica

I want to solve n ODE in Mathematica, but there is an error I cannot understand: "The function x appears with no arguments."

This is the code I used:

eqn = x''[y] + a x'[y] Sin[y - y0] + a x[y] (Cos[y - y0] - s) == 0;

sol = DSolve[eqn, x, y]


Could anyone help me? Is there another function that solve the problem?

• Your eqn should read eqn = x''[y] + a x'[y] Sin[y - y0] + a x[y] (Cos[y - y0] - s) == 0; However, DSolve cannot handle this equation. – Bob Hanlon Nov 7 '16 at 5:52
• Thank you, @BobHanlon . So which function should I use? – sara nj Nov 7 '16 at 6:17

DSolve is going to have issues with your ode, I doubt that there is a closed form answer. You might look instead at using NDSolve and settle for a numerical approximation. Notice that this will force you to pick concrete values for $y_0$ in order to run numerical solvers.

Alternatively, you could linearize the equation and use DSolve to capture leading order behavior (just like we approximate a pendulum by $y'' + y = 0$ instead of $y'' + \sin y = 0$). Techniques exist to extend this method to capture higher order terms instead of just linearizing.

Continuation of the answer of @erfink: Use NDSolve and see what influence parameters have.

 eqn[y0_, xs_, x0_, s_, a_] =
x''[y] + a x'[y] Sin[y - y0] + a x[y] (Cos[y - y0] - s) == 0 &&
x' == xs && x == x0;

xsol[z_, y0_, xs_, x0_, s_, a_, b_,
c_] := (x[z] /. First@NDSolve[eqn[y0, xs, x0, s, a], x, {y, b, c}])

Manipulate[
Plot[xsol[z, y0, xs, x0, s, a, -3, 3], {z, -3, 3}], {{y0, 0}, -3,
3}, {{xs, 0}, -3, 3}, {{x0, 1}, -3, 3}, {{s, 0}, -3,
3}, {{a, 1}, -3, 3}]


Attention: Use z instead of y. Otherwise you get problems with the y in NDSolve.