```Date: Feb 22, 2014 3:01 AM
Author: Daniel Stariolo
Subject: Some linear constraints seem to be ignored in function NMinimize with

I'm trying to minimize a non-linear function of four variables with some linear constraints. Mathematica 8 is unable to find a good solution giving complex values of the function at some point in the iteration. This implies that one or some contraints are not being enabled in the process. Is this a bug or limitation of the optimization function ?Function to minimize isff[lxw_, lwz_, c_, d_] := - J1 (lxw + lwz) - 2 J2 c +  T (-Log[2] - 1/2 (1 - lxw) Log[(1 - lxw)/4] -     1/2 (1 + lxw) Log[(1 + lxw)/4] -     1/2 (1 - lwz) Log[(1 - lwz)/4] -     1/2 (1 + lwz) Log[(1 + lwz)/4] + 1/2 (1 - d) Log[(1 - d)/16] +     1/8 (1 + 2 c + d - 2 lwz - 2 lxw) Log[       1/16 (1 + 2 c + d - 2 lwz - 2 lxw)])whereT = 10;J1 = 1;J2 = -0.2;are constant parameters. Then I tryNMinimize[{ff[lxw, lwz, c, d],  2 c + d - 2 lwz - 2 lxw  >= -0.999 && -0.999 <= lxw <=   0.999 && -0.999 <= lwz <= 0.999 && -0.999 <= c <= 0.999 &&  d <= 0.9999}, {lxw, lwz, c, d}]with the resultNMinimize::nrnum: "The function value 5.87777-4.87764 I is not a real number at {c,d,lwz,lxw} = {-0.718817,-1.28595,0.69171,-0.932461}."I would appreciate if someone can give a hint at what is happening here.
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