MathGroup Archive 2001

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restrictions on parameters


Example: Given the restrictions on the parameters as shown below, I
would like to check the sign of the determinant or the eigenvalues of
Matrix A. The problem seems to be that the restricions on the parameters
are not properly specified.

Clear[a, b, c, d]
a < 0; b > 0; c > 0; d > 0;
A = {{a, b}, {c, d}};

eigen = Eigensystem[A];
{d1, d2} = {eigen[[1, 1]], eigen[[1, 2]]};

det1 = Det[A]
-b c + a d

TrueQ[det1 < 0]
False

This should be true!

TrueQ[d1 < 0]
False


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