Solution
= Solution
Write each unrestricted coordinate as $x_j=u_j-v_j$ with $u_j,v_j\ge0$. Then the primal has nonnegative variables $(u,v)$, constraint <matrix> $[A,-A]$, and objective coefficients $(c,-c)$. Its dual constraints are $A^Ty\ge c$ and $-A^Ty\ge-c$, hence equality. Therefore
$$
\boxed{D^*:\ \min b^Ty\text{ subject to }A^Ty=c,\ y\ge0.}
$$
The removed sign restriction on the primal variables becomes equality of the dual column constraints.