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I have a system of equations I want to solve using NDSolve, which is

NDSolve[{Q''[t] + 3 H*Q'[t] + ((H)^2 + 3 xt^2 - xt^4/(2 H^2) + 2 xt*Vt/H + Vtt) Q[t] == 0, Abs[Q[t65]] == 149.11962399146714`, Abs[Q'[t65]] == 73.73487138233784`}, Q, {t, t65, t55}];

Every function in there is real-valued, except for Q[t], which should be complex. However, I am trying to see if I could use the Abs function in the initial condition, since it could be simpler for me. However, the solution for Q[t] looks like this:

enter image description here

Which should not be the case, since I am expecting a function that highly oscillates when $t$ approaches zero, and remains constant after sometime. So, is there a solution to use the function Abs[] in here?

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    $\begingroup$ Please post the full code. $\endgroup$
    – cvgmt
    Commented May 13 at 11:02
  • $\begingroup$ If you specify the absolute value of Q[t], this doesn't completely specify the initial condition. I don't see why you'd want to do this. Mathematica is likely assuming that Q[t65] is real and proceeding from there. Also, please include the numerical values in the differential equation you used to generate the figure. $\endgroup$
    – march
    Commented May 13 at 17:16

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