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I'm testing the FindInstance capabilities and ran into one problem that behaves similarly with different types of inequalities. I calculate two points satisfying given inequalities:

FindInstance[Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2] > 0 && Subscript[X, 1]^2 - 4 Subscript[X, 0] > 0 && Subscript[Y, 1]^2 - 4 Subscript[Y, 0] > 0 && Subscript[Z, 1]^2 - 4 Subscript[Z, 0] > 0 && -5 < Subscript[W, 2] <5 && -2 < Subscript[X, 1] < 2 && 1 < Subscript[Y, 1] < 2 && 1 < Subscript[Z, 1] < 2, Reduce`FreeVariables[{Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2], Subscript[X, 1]^2 - 4 Subscript[X, 0], Subscript[Y, 1]^2 - 4 Subscript[Y, 0],Subscript[Z, 1]^2 - 4 Subscript[Z, 0], Subscript[W, 1] + Subscript[W, 2] (Subscript[S, 1])}], Reals, 2]

And when I add an additional inequality $-1 < W_1 + W_2 S_1 < 1$

FindInstance[Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2] > 0 && Subscript[X, 1]^2 - 4 Subscript[X, 0] > 0 && Subscript[Y, 1]^2 - 4 Subscript[Y, 0] > 0 && Subscript[Z, 1]^2 - 4 Subscript[Z, 0] > 0 && -5 < Subscript[W, 2] <5 && -2 < Subscript[X, 1] < 2 && 1 < Subscript[Y, 1] < 2 && 1 < Subscript[Z, 1] < 2 && -1 < Subscript[W, 1] + Subscript[W, 2] (Subscript[S, 1]) < 1, Reduce`FreeVariables[{Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2], Subscript[X, 1]^2 - 4 Subscript[X, 0], Subscript[Y, 1]^2 - 4 Subscript[Y, 0], Subscript[Z, 1]^2 - 4 Subscript[Z, 0], Subscript[W, 1] + Subscript[W, 2] (Subscript[S, 1])}], Reals, 2];

The calculation starts but cannot stop, i.e. identify these 2 points. I waited an hour, but the calculation never ended.

Although with the help of the NMinimize command, I can find one point quite quickly.

NMinimize[{Subscript[W, 1] + Subscript[W, 2] Subscript[S, 1], Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2] > 0, Subscript[X, 1]^2 - 4 Subscript[X, 0] > 0, Subscript[Y, 1]^2 - 4 Subscript[Y, 0] > 0, Subscript[Z, 1]^2 - 4 Subscript[Z, 0] > 0, -1 < Subscript[W, 1] + Subscript[W, 2] Subscript[S, 1] < 1}, Reduce`FreeVariables[{Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2], Subscript[X, 1]^2 - 4 Subscript[X, 0], Subscript[Y, 1]^2 - 4 Subscript[Y, 0], Subscript[Z, 1]^2 - 4 Subscript[Z, 0], Subscript[W, 1] + Subscript[W, 2] (Subscript[S, 1])}], Method -> {"RandomSearch", "SearchPoints" -> 1}]

Is there something wrong with the FindInstance settings?

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3 Answers 3

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FindInstance depends on the order of variables. In your example, you can reorder the variables (as Bob does in his answer) to get things to work:

expr = Subscript[W,1]^2 - 4 Subscript[W,0] Subscript[W,2] > 0 &&
    Subscript[X,1]^2-4 Subscript[X,0]>0 && 
    Subscript[Y,1]^2-4 Subscript[Y,0]>0 && 
    Subscript[Z,1]^2-4 Subscript[Z,0]>0 &&
    -5<Subscript[W,2]<5 &&
    -2<Subscript[X,1]<2 &&
    1<Subscript[Y,1]<2 &&
    1<Subscript[Z,1]<2 &&
    -1<Subscript[W,1]+Subscript[W,2] (Subscript[S,1])<1;

FindInstance[expr, RotateLeft @ Reduce`FreeVariables[expr], Reals, 2]

{{Subscript[W, 0] -> 0, Subscript[W, 1] -> -(9/202), Subscript[W, 2] -> 26/7, Subscript[X, 0] -> -96, Subscript[X, 1] -> -(24/13), Subscript[Y, 0] -> 1/4, Subscript[Y, 1] -> 98/51, Subscript[Z, 0] -> -105, Subscript[Z, 1] -> 45/34, Subscript[S, 1] -> -(35/188)}, {Subscript[W, 0] -> 0, Subscript[W, 1] -> -(9/202), Subscript[W, 2] -> 26/7, Subscript[X, 0] -> 19/202, Subscript[X, 1] -> -(88/73), Subscript[Y, 0] -> 163/269, Subscript[Y, 1] -> 205/114, Subscript[Z, 0] -> -105, Subscript[Z, 1] -> 45/34, Subscript[S, 1] -> -(35/188)}}

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  • $\begingroup$ Similar problem. As soon as I replace replace S[1] with X[1]+Y[1]+Z[1] for example, the problem remains. $\endgroup$
    – dtn
    Jan 14 at 3:34
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It is generally recommended to avoid subscripted variables and instead to use indexed variables. The indexed variables can be formatted to display as the corresponding subscripted variables.

Clear["Global`*"];

SeedRandom[1234];

(Format[#[n_]] := Subscript[#, n]) & /@ {S, W, X, Y, Z};

sys1 = W[1]^2 - 4 W[0] W[2] > 0 && -4 X[0] + X[1]^2 > 0 && -4 Y[0] + Y[1]^2 > 
    0 && -4 Z[0] + Z[1]^2 > 0 && -5 < W[2] < 5 && -2 < X[1] < 2 && 
   1 < Y[1] < 2 && 1 < Z[1] < 2;

vars1 = Reduce`FreeVariables[sys1];

FindInstance[sys1, vars1, Reals, 2]

enter image description here

sys2 = sys1 && -1 < W[1] + W[2] S[1] < 1;

vars2 = {vars1, S[1]} // Flatten;

FindInstance[sys2, vars2, Reals, 2]

enter image description here

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  • $\begingroup$ Your method was the fastest. But if we replace S[1] with X[1]+Y[1]+Z[1] for example, the problem remains. He can't even count two points. $\endgroup$
    – dtn
    Jan 14 at 3:29
  • $\begingroup$ Replacing S[1] with (X[1]+Y[1]+Z[1]) there appears to be only one instance. Using varying values for RandomSeeding returns the same solution: {Subscript[W, 0] -> -(19/21), Subscript[W, 1] -> -(19/84), Subscript[W, 2] -> 0, Subscript[X, 0] -> -(19/21), Subscript[X, 1] -> 95/168, Subscript[Y, 0] -> 0, Subscript[Y, 1] -> 19/12, Subscript[Z, 0] -> 19/84, Subscript[Z, 1] -> 19/14} $\endgroup$
    – Bob Hanlon
    Jan 14 at 3:55
  • $\begingroup$ expr = Subscript[W, 1]^2 - 4 Subscript[W, 0] Subscript[W, 2] > 0 && Subscript[X, 1]^2 - 4 Subscript[X, 0] > 0 && Subscript[Y, 1]^2 - 4 Subscript[Y, 0] > 0 && Subscript[Z, 1]^2 - 4 Subscript[Z, 0] > 0 && -5 < Subscript[W, 2] <5 && -2 < Subscript[X, 1] < 2 && 1 < Subscript[Y, 1] < 2 && 1 < Subscript[Z, 1] < 2 && Subscript[W, 1] + (Subscript[X, 1] + Subscript[Y, 1] + Subscript[Z, 1]) < 100; /FindInstance[expr, RotateLeft@Reduce`FreeVariables[expr], Reals, 2]/ Here I put the new inequality. Two points were counted, but the calculation lasted about 15 minutes - this is a fabulous lot. $\endgroup$
    – dtn
    Jan 14 at 4:01
  • $\begingroup$ Addition: for comparison, I used the SemialgebraicComponentInstances command. Two points were found instantly. But I do not like the functionality of this command and the lack of the ability to select the number of points. Something is wrong with the FindInstance command. sys2 = sys1 && -1 < W[1] + X[1] + Y[1] + Z[1] < 1; vars2 = {vars1, S[1]} // Flatten; SemialgebraicComponentInstances[sys2, vars2] $\endgroup$
    – dtn
    Jan 14 at 4:12
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Another way is Reduce the expr before use FindInstance.

FindInstance[Reduce[expr, Reals], Reduce`FreeVariables[expr], Reals,2]

enter image description here

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1
  • $\begingroup$ Similar problem. As soon as I replace replace S[1] with X[1]+Y[1]+Z[1] for example, the problem remains. It seems that the problem is with the structure of the inequalities themselves and the boundary conditions. $\endgroup$
    – dtn
    Jan 14 at 3:34

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