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I'm trying to understand how to express a simple LP in the standard semidefinite programming (SDP) form.

In particular, consider the following linear program: $$ \begin{aligned} \min_{x_1, x_2} \;& -2x_1 - x_2 \\ \text{s.t. } & \begin{cases} -x_1 + x_2 \le 1,\\ x_1 - 2x_2 \le 2,\\ x_1 \ge 0,\\ x_2 \ge 0. \end{cases} \end{aligned} $$ I know that a general SDP can be written as:
As $F_i$ symmetric matrix and ⪯0 (non positive definitive)
How can this LP be represented in that matrix form?

What I know, that I first should do: $$ \begin{aligned} \min_{x_1, x_2} \;& -2x_1 - x_2 \\ \text{s.t. } & \begin{cases} -x_1 + x_2 - 1 \le 0,\\ x_1 - 2x_2 - 2 \le 0,\\ -x_1 \le 0,\\ -x_2 \le 0. \end{cases} \end{aligned} $$ So, from the form: $$ \min_x \; c^T x \quad \text{s.t. } \quad F(x) = F_0 + \sum_i x_i F_i \preceq 0. $$

Sadly my professor didn't explain this very well.. I tried to learn using google, but I don't think I understand how to do it... I created 4 matrices of the diagonal, but I need to have it summed or something like that.

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  • $\begingroup$ Related $\endgroup$ Commented Nov 30 at 10:38
  • $\begingroup$ @RodrigodeAzevedo I don't think I need to relate it to spectrahedron. At the lecture, they didn't say something regarding this, she just gave examples and that is it. if I will do something like this at the test, it will take me forever. $\endgroup$ Commented Dec 1 at 16:15
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    $\begingroup$ Ben, by definition, the feasible region of an SDP is a spectrahedron. All you have to do is use a diagonal matrix (whose diagonal entries are affine polynomials of the form $a_i x_1 + b_i x_2 - c_i$) and the $F_i$ matrices are the diagonal matrices containing the coefficients of $x_i$ and also the constants $c_i$. This is all done in the answers to the question linked to above $\endgroup$ Commented Dec 1 at 16:41
  • $\begingroup$ @RodrigodeAzevedo I will try this later and update, thanks :) $\endgroup$ Commented Dec 1 at 19:45

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