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When I use the below script to solve an equation and plot the solution, the plot shows some discontinuous lines. For example at point $x\approx0.25$ the solver cannot find the answer and the plot is discontinuous. How can I fix this problem?

Clear["Global`*"]

c = -0.8;
L = 1;
b = Pi/L;

{x1, x2} = {0, L};

eqn = f[x] == c Sin[2 f[x]] + b x + Pi/2;

f[x_] = f[x] /. 
   DSolve[{D[eqn, x], 
      f[0] == (f0 /. 
         Solve[(eqn /. x -> 0 /. f[0] -> f0), f0, Reals][[1]])}, 
     f[x], {x, x1, x2}][[1]] // FullSimplify

(*InverseFunction[-(1/2) Cos[2 #1]+#1&][(11 x)/10]*)

Plot[f[x], {x, x1, x2}]
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  • $\begingroup$ Plot does not plot complex numbers. With your code f[0.1]=1.09728 -0.320829 I. You can choose to plot the real part with Re[f[x]] $\endgroup$
    – Bill Watts
    Commented Jun 27, 2020 at 20:38

1 Answer 1

Reset to default
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$Version

(* "12.1.1 for Mac OS X x86 (64-bit) (June 19, 2020)" *)

Clear["Global`*"]

c = -4/5;
L = 1;
b = Pi/L;
{x1, x2} = {0, L};

eqn = f[x] == c Sin[2 f[x]] + b x + Pi/2;

The equation does not identify a unique initial condition

ic = f0 /. Solve[(eqn /. x -> 0 /. f[0] -> f0), f0, Reals]

enter image description here

Only one of the possible initial conditions leads to a solution

sol = Table[
   DSolve[{D[eqn, x], f[0] == ic[[n]]}, f[x], {x, x1, x2}] // 
    FullSimplify, {n, 1, 3}] // Quiet

enter image description here

f[x_] = f[x] /. sol[[2, 1]];

Not all values of x produce real results

f[0.1`15]

(* 1.0972751444995646 - 0.3208287706939249 I *)

Plotting,

Plot[f[x], {x, x1, x2},
 PlotRange -> {0, 5.8},
 WorkingPrecision -> 15,
 Exclusions -> All,
 PlotPoints -> 50,
 MaxRecursion -> 2]

enter image description here

Improve this answer
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  • $\begingroup$ I had tried to find coefficients $c_1$ and $c_2$ myself so that $\sin(2f(x=0))=\sin(2f(x=L))$ and also $\cos(2f(x=0))=\cos(2f(x=L))$ $\endgroup$
    – Alex Stark
    Commented Jun 28, 2020 at 3:25
  • $\begingroup$ Do not ask new questions in comments. Organize your thoughts and post a new question with all pertinent information. Then anyone who is interested will see the new question. $\endgroup$
    – Bob Hanlon
    Commented Jun 28, 2020 at 3:38

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