I have a numerical solution for a differential equation. I know that the gray dotted solution is unphysical. Is there a way I can tell mathematica to ignore the gray dotted solution (e.g. restrict the interpolating function to values between 0 and 1)? (Later on, I want to do operations on this solution, and I don't want the gray solution to contribute) enter image description here

I tried to use WhenEvent, but I get an error message.

sa = NDSolve[{3/2 (a'[t]/a[t])^2 + 
     a'[t]/a[t] vdot[t]/v[t] - ωa/8 vdot[t]^2 == 
    1/4 ρ0/a[t]^3, a[todaya] == 1, 
   WhenEvent[a[t] > 1, "StopIntegration"]}, a, {t, EQfraca, todaya}]

aa = Plot[Evaluate[a[t] /. sa], {t, EQfraca, todaya}, 
   PlotRange -> {{EQfraca, todaya}, {mina, maxa}}, Frame -> True, 
   PlotStyle -> {{Red, Thick}, {Gray, Dotted}}, 
   FrameLabel -> {"t", "a (t)"}];
  • $\begingroup$ Hi! You can format inline code and code blocks by selecting the code and clicking the {} button above the edit window. The edit window help button ? is also useful for learning how to format your questions and answers. You may also find this this meta Q&A helpful $\endgroup$ – Michael E2 Aug 18 '15 at 20:56
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  • 1
    $\begingroup$ Many symbols are undefined in the code in your question. $\endgroup$ – bbgodfrey Aug 18 '15 at 21:33

In the absence of definitions for several functions and constants, I chose

v[t] = 1; vdot[t] = 1; todaya = .8; EQfraca = .5; ωa = 1; ρ0 = 1; mina = 0.9; maxa = 1.3;

NDSolve then yields two InterpolatingFunctions

(* {{a -> InterpolatingFunction[{{0.5, 0.8}}, <>]}, 
    {a -> InterpolatingFunction[{{0.5, 0.8}}, <>]}} *)

which in turn yields two curves.

enter image description here

(Note that I changed PlotStyle to {{Gray, Dotted}, {Red, Thick}} for consistency with the figure in the question.) Because the two InterpolatingFunctions are distinct, simply do not use the one you do not want.

By the way, WhenEvent[a[t] > 1, "StopIntegration"] has no effect, because the upper curve starts at a[t] ==1, so no crossing occurs. To trigger it, use a[t] > 1.000001, for instance. This does not eliminate Gray curve, but it does make it very short.

Addendum: In answer to a comment below, the InterpolatingFunction which is less than 1 can be picked out by, for instance,

If[First@(a[todaya - .01] /. sa) < 1, First@sa, Last@sa]
| improve this answer | |
  • $\begingroup$ Got it. Thanks. =) I wrote {say,sa2}= NDSolve[....] sa1 covers one solution, say2 covers the other solution. $\endgroup$ – Bob Aug 18 '15 at 22:16
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    $\begingroup$ Yes, that works. If you wish to automate the process, try something like sa[[Which[First@(a[.7] /. sa) < 1, 1, Last@(a[.7] /. sa) < 1, 2]]] $\endgroup$ – bbgodfrey Aug 18 '15 at 22:22

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