yb[a_?NumericQ] := a^2;
eqns = {x'[s] == -yb[s], WhenEvent[x'[s] > 0, {yb[s] -> - yb[s]}], 
  x'[0] == 0}
NDSolve[eqns, x[s], {s, -50, 50}]

The WhenEvent doesn't evaluate x'[s] correctly, is there any way to make this work?

  • 1
    $\begingroup$ Is this an example or a real problem? $\endgroup$ – Alex Trounev Jan 3 at 20:06
  • $\begingroup$ this is example,but actually I need to evaluate the sign of first derivative when solving first-order partial differential equation... $\endgroup$ – Xminer Jan 3 at 20:10
  • $\begingroup$ The problem is incorrect, so you need to change the question. Add a real example. $\endgroup$ – Alex Trounev Jan 3 at 20:24

You cannot change the functiondefinition yb[a] inside a WhenEvent!


eqns = {x'[s] == - sgn[s] yb[s], sgn[0] == +1,WhenEvent[x'[s] > 0, {sgn[s] -> -sgn[s] }], x'[0] == 0}
{X, SGN} =NDSolveValue[eqns, {x, sgn} , {s, -50, 50}, DiscreteVariables -> sgn]
  • $\begingroup$ Thanks,this seems perfect for my problem. May I ask some questions for study? 1) why cannot change yb[a] inside WhenEvent? 2) why we need to introduce sgn function into the equation and specify it for DiscreteVariables Option? $\endgroup$ – Xminer Jan 3 at 20:35
  • 3
    $\begingroup$ yb[s] is an extern function , WhenEvent can only change state variables of NDSolve. $\endgroup$ – Ulrich Neumann Jan 3 at 20:43

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