# Semidefinite Programming

The primal standard form of a conic program with $n_{s}$ symmetric positive semidefinite cones

$$\mathbb{S}^{s_{j}}_{+} := \left\{ X \in \mathbb{R}^{s_{j} \times s_{j}} \colon\; X = X^{T},\; v^{T} X v \geq 0,\; \forall v \in \mathbb{R}^{s_{j}} \right\},\quad j = 1,\ldots,n_{s}.$$

is

$$\begin{array}{lll} \text{minimize} & \sum_{j=1}^{n_{s}} \langle C_{j}, X_{j} \rangle & \\ \text{subject to} & \sum_{j=1}^{n_{s}} \langle A_{ij}, X_{j} \rangle = b_{i}, & i = 1,\ldots,m, \\ & X_{j} \in \mathbb{S}^{s_{j}}_{+}, & j = 1,\ldots,n_{s}, \end{array}$$

with symmetric $s_{j} \times s_{j}$ matrices $A_{ij}$ and $C_{j}$. The dual problem form is

$$\begin{array}{ll} \text{maximize} & b^{T} y \\ \text{subject to} & Z_{j} := C_{j} - \sum_{i=1}^{m} y_{i} A_{ij} \in \mathbb{S}^{s_{j}}_{+},\quad j = 1, \ldots, n_{s}. \end{array}$$

## A feasible SDP

We consider an example from the CSDP User’s Guide [Borchers2017]:

$$\begin{array}{lll} \text{minimize} & \sum_{j=1}^{3} \langle C_{j}, X_{j} \rangle & \\ \text{subject to} & \sum_{j=1}^{3} \langle A_{ij}, X_{j} \rangle = b_{i},\quad i = 1,2, \\ & X_{1} \in \mathbb{S}^{2}_{+}, \\ & X_{2} \in \mathbb{S}^{3}_{+}, \\ & X_{3} \in \mathbb{S}^{2}_{+}, \end{array}$$

where $b = \begin{pmatrix} 1 \ 2 \end{pmatrix}$,

$$\begin{array}{ccc} C^{s_{1}}_{1} = \begin{pmatrix} -2 & -1 \\ -1 & -2 \end{pmatrix}, & C^{s_{2}}_{2} = \begin{pmatrix} -3 & 0 & -1 \\ 0 & -2 & 0 \\ -1 & 0 & -3 \end{pmatrix}, & C^{s_{3}}_{3} = \begin{pmatrix} 0 & 0 \\ 0 & 0 \end{pmatrix}, \\ A^{s_{1}}_{1,1} = \begin{pmatrix} 3 & 1 \\ 1 & 3 \end{pmatrix}, & A^{s_{2}}_{1,2} = \begin{pmatrix} 0 & 0 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0 \end{pmatrix}, & A^{s_{3}}_{1,3} = \begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix}, \\ A^{s_{1}}_{2,1} = \begin{pmatrix} 0 & 0 \\ 0 & 0 \end{pmatrix}, & A^{s_{2}}_{2,2} = \begin{pmatrix} 3 & 0 & 1 \\ 0 & 4 & 0 \\ 1 & 0 & 5 \end{pmatrix}, & A^{s_{3}}_{2,3} = \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix}. \end{array}$$

In the vectorized format the corresponding coefficient matrix At and the primal objective vector c are

And the cone structure K for this problem is

Now we compute approximate solutions by using solve and then verified error bounds by using rigorous_lower_bound and rigorous_upper_bound:

Finally, we get an overview about all the performed computations:

To compare the approximate solution X, y, and Z with [Borchers2017] the vectorized solution quantities x and z have to be transformed back to matrices by using vsdp.smat and the appropriate scaling factor:

The compuation of the rigorous lower bounds involves the computation of the smallest eigenvalues Zl(j)= $\lambda_{\min}([Z_{j}])$ for $j = 1,2,3$.

Since all Zl >= 0 it is proven that all matrices $Z_{j}$ are in the interior of the cone $\mathbb{S}^{2}{+} \times \mathbb{S}^{3}{+} \times \mathbb{S}^{2}_{+}$ and Y is a rigorous enclosure of a dual strict feasible (near optimal) solution.

Analogous computations are performed for the rigorous upper bound. Here lower bounds on the smallest eigenvalue of the primal solution are computed Xl(j)= $\lambda_{\min}([X_{j}])$ for $j = 1,2,3$.

The matrix X is a rigorous enclosure of a primal strict feasible (near optimal) solution and can be restored from the vectorized quantity obj.solutions.rigorous_upper_bound.x as shown for the approximate solution. We omit the dispay of the interval matrix X for brevity.

Since all Xl are positive, strict feasibility for the primal problem is proved. Thus strong duality holds for this example.

## An infeasible SDP

Now we consider the following example (see [Jansson2007a]):

$$\begin{array}{ll} \text{minimize} & \langle C(\delta), X \rangle \\ \text{subject to} & \langle A_{1}, X \rangle = 1, \\ & \langle A_{2}, X \rangle = \epsilon, \\ & \langle A_{3}, X \rangle = 0, \\ & \langle A_{4}, X \rangle = 0, \\ & X \in \mathbb{S}^{3}_{+}, \end{array}$$

with Lagragian dual

$$\begin{array}{ll} \text{maximize} & y_{1} + \epsilon y_{2} \\ \text{subject to} & Z(\delta) := C(\delta) - \sum_{i = 1}^{4} A_{i} y_{i} \in \mathbb{S}^{3}_{+}, \\ & y \in \mathbb{R}^{4}, \end{array}$$

where

The linear constraints of the primal problem form imply

$$X(\epsilon) = \begin{pmatrix} \epsilon & -1 & 0 \\ -1 & X_{22} & 0 \\ 0 & 0 & X_{33} \end{pmatrix} \in \mathbb{S}^{3}_{+}$$

iff $X_{22} \geq 0$, $X_{33} \geq 0$, and $\epsilon X_{22} - 1 \geq 0$. The conic constraint of the dual form is

$$Z(\delta) = \begin{pmatrix} -y_{2} & \frac{1+y_{1}}{2} & -y_{3} \\ \frac{1+y_{1}}{2} & \delta & -y_{4} \\ -y_{3} & -y_{4} & \delta \end{pmatrix} \in \mathbb{S}^{3}_{+}.$$

Hence, for

• $\epsilon \leq 0$: the problem is primal infeasible $\hat{f_{p}} = +\infty$.
• $\delta \leq 0$: the problem is dual infeasible $\hat{f_{d}} = -\infty$.
• $\epsilon = \delta = 0$: the problem is ill-posed and there is a duality gap with $\hat{f_{p}} = +\infty$ and $\hat{f_{d}} = -1$.
• $\epsilon > 0$ and $\delta > 0$: the problem is feasible with $\hat{f_{p}} = \hat{f_{d}} = -1 + \delta / \epsilon$.

We start with the last feasible case and expect $\hat{f_{p}} = \hat{f_{d}} = -1 + 10$ with.

Now we change the setting for primal infeasiblilty, what SDPT3 detects as well:

The value of the return parameter info confirms successful termination of the solver. The first eight decimal digits of the primal and dual optimal values are correct, since $\hat{f}{p} = \hat{f}{d} = -0.5$, all components of the approximate solutions xt and yt have at least five correct decimal digits. Nevertheless, successful termination reported by a solver gives no guarantee on the quality of the computed solution.

For instance, if we apply SeDuMi to the same problem we obtain:

SeDuMi terminates without any warning, but some results are poor. Since the approximate primal optimal value is smaller than the dual one, weak duality is not satisfied. In other words, the algorithm is not backward stable for this example. The CSDP-solver gives similar results:

A good deal worse are the results that can be derived with older versions of these solvers, including SDPT3 and SDPA [Jansson2006].

Reliable results can be obtained by the functions vsdplow and vsdpup. Firstly, we consider vsdplow and the approximate solver SDPT3.

the vector y is a rigorous interior dual $??$-optimal solution where we shall see that $?? \approx 2.27 \times 10^{-8}$. The positivity of dl verifies that y contains a dual strictly feasible solution. In particular, strong duality holds. By using SeDuMi similar rigorous results are obtained. But for the SDPA-solver we get

Thus, an infinite lower bound for the primal optimal value is obtained and dual feasibility is not verified. The rigorous lower bound strongly depends on the computed approximate solution and therefore on the used approximate conic solver.

Similarly, a verified upper bound and a rigorous enclosure of a primal $??$-optimal solution can be computed by using the vsdpup function together with SDPT3:

The output fU is close to the dual optimal value $\hat{f}_{d} = -0.5$. The interval vector x contains a primal strictly feasible solution, see \eqref{OptSolSDPExp}, and the variable lb is a lower bound for the smallest eigenvalue of x. Because lb is positive, Slater’s condition is fulfilled and strong duality is verified once more.

Summarizing, by using SDPT3 for the considered example with parameter $?? = 10^{-4}$, VSDP verified strong duality with rigorous bounds for the optimal value $% -0.500000007 \leq \hat{f}_{p} = \hat{f}_{d} \leq -0.499999994. %]]>$

The rigorous upper and lower error bounds of the optimal value show only modest overestimation. Strictly primal and dual feasible solutions are obtained. Strong duality is verified. Moreover, we have seen that the quality of the rigorous results depends strongly on the quality of the computed approximations.

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