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I am new to Mathematica. I just want to evaluate a 2x2 Jacobian and then evaluate it on different values of the two variables x and y.

The values I want to use to evaluate the Jacobian are the result of NDSolve.

Code:

f[x_, y_] := 2 - 10 x^2; g[x_, y_] := 10 y - 2 y^2; sol = NDSolve[{x'[t] == f[x[t], y[t]], y'[t] == g[x[t], y[t]], x[0] == 1, y[0] == 1}, {x, y}, {t, 0, 10}]; JacobianMatrix[f_List?VectorQ, x_List] := Outer[D, f, x] /; Equal @@ (Dimensions /@ {f, x}); lx = Flatten[Map[x, Range[0, 10, 0.1]] /. sol]; ly = Flatten[Map[y, Range[0, 10, 0.1]] /. sol];

Now I want to evaluate the 

JacobianMatrix[{f[x, y], g[x, y]}, {x, y}]

on the values of lx and ly (no combinations), just take one value of each one and produce a Jacobian.

I've tried Map and Table and combinations with no result.

Any help is welcomed. Thanks!

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  • $\begingroup$ also tried Evaluate and @@ $\endgroup$ Commented Mar 19, 2020 at 0:46
  • $\begingroup$ I am trying with few values to try to make it work, for example JacobianMatrix[{f[x, y], g[x, y]}, {x, y}] /@ {{0, 1}} with no luck. $\endgroup$ Commented Mar 19, 2020 at 0:54
  • $\begingroup$ this is working but not giving the intended result: JC[{xx_, yy_}] := JacobianMatrix[{f[x, y], g[x, y]}, {x, y}] /. {x -> xx, y -> yy} JC /@ {{lx, ly}} Because I need a list of Jacobian matrices as result. $\endgroup$ Commented Mar 19, 2020 at 1:02
  • $\begingroup$ this is not working: JC[xx_, yy_] := JacobianMatrix[{f[x, y], g[x, y]}, {x, y}] /. {x -> xx, y -> yy} Map[JC, {{lx,ly}}] $\endgroup$ Commented Mar 19, 2020 at 1:05
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    $\begingroup$ Why didn't you fix the error by editing the question? $\endgroup$ Commented Mar 19, 2020 at 1:12

1 Answer 1

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Method 1:

ans = Transpose[ Map[Thread, D[{f[x, y], g[x, y]}, {{x, y}}] /. {x -> lx, y -> ly}, 2], {2, 1, 3}]; MatrixForm /@ ans

Method 2:

ans = MapThread[ Function[{x, y}, Evaluate@D[{f[x, y], g[x, y]}, {{x, y}}]], {lx, ly}];

Answer to original question

Your Jacobian matrix is constant. There's no need to evaluate it at many different points.

JacobianMatrix[{f[x, y], g[x, y]}, {x, y}] (* {{-10, 0}, {0, 8}} *)

Note a common way to compute the Jacobian matrix is (search for Jacobian the docs for D):

D[{f[x, y], g[x, y]}, {{x, y}}]
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  • $\begingroup$ sorry, I've simplified the original problem to keep it simple and it was too much of a simplification... now I've set y^2 and x^2 the Jacobian is not constant. thanks. $\endgroup$ Commented Mar 19, 2020 at 1:21
  • $\begingroup$ Thanks! It works perfectly. I've tried for hours with no luck... Mathematica learning curve is a bit steep... $\endgroup$ Commented Mar 19, 2020 at 4:19
  • $\begingroup$ @CharlesM You're welcome. :) $\endgroup$ Commented Mar 19, 2020 at 12:48
  • $\begingroup$ I think what I've found difficult was to distinguish between the different types of objects that mathematica can handle. List, List of lists, Matrix, etc $\endgroup$ Commented Mar 19, 2020 at 18:29

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