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restrictions on parameters

• To: mathgroup at smc.vnet.net
• Subject: [mg31955] restrictions on parameters
• From: steger at uni-greifswald.de (Thomas Steger)
• Date: Fri, 14 Dec 2001 04:21:18 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

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