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I need to use NIntegrate inside NDSolve, for example:

NDSolve[{y'[x] == x + NIntegrate[r, {r, 1, y[x]}], y[2] == 0.5}, y, {x, 0, 1}]

How can I make this work?

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    $\begingroup$ Why not a. evaluate the integral explicitly; or b. differentiate again so that you have a true ODE instead of an integro-differential equation? $\endgroup$ Jun 7, 2016 at 19:09

1 Answer 1

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This may be a perverse way of solving the problem as stated, but as general technique it may be useful. Define an auxiliary function that is only evaluated for numeric arguments, e.g.

f[y_?NumericQ] := NIntegrate[r, {r, 1, y}]

m = NDSolve[{y'[x] == x + f[y[x]], y[2] == 0.5}, y, {x, 0, 1}]
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  • $\begingroup$ How should I define coefficients of ODE including NIntegrate if ODE is intended for ParametricNDSolveValue? So far I tried someting like A1N[k_?IntegerQ, bv_?NumericQ, p_?NumericQ] := bva2D1N[k, bv, p]/bva2N[k, bv, p]; where p is free parameter and bv is substituted by bv[z] in final form of ODE for function phi[z]. All functions with names ended by N are some call to NIntegrate. ` $\endgroup$ Sep 3, 2022 at 11:06
  • $\begingroup$ I suggest you ask this as a question where it will be seen by more people. $\endgroup$
    – mikado
    Sep 3, 2022 at 11:29

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