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In the wikipedia page of LMI, the standard form is given by

$$A_0+y_1A_1+y_2A_2+\cdots+y_mA_m \succeq 0,$$

where $A_i$ are $m\times m$ symmetric matrices and $y_i$ are real vectors, $i=1,2,\ldots m.$

I have few problems in this LMI standard form that I would like to solve. I know $A_i's$ and want to find if there exist $y_i's$ that satisfy the above inequality. So it is a feasibility problem. Sometimes I require some $y_i$ to take only positive values.

However, when searching for LMI Matlab or CVX solvers, I always find some other forms of LMI, like $A^T P + P A \prec 0$ ex1, which I cannot connect with standard form from wikipedia.

Is it compulsory to rewrite standard form of LMI to be able to solve it? I wonder if there is ready Matlab or cvx programs that can solve standard form.


Example: Given $A\in\mathbb{C}^{n\times n}$, check if there exists $k\in\mathbb{R}$ and $\alpha\in[0,\pi]$, such that

$$(A^*A-I)+\frac{k}{2}(A+A^*)+k\cos\alpha I \succ 0,$$

Let $A_0=A^*A-I$, $A_1=A+A^*$, $A_2=I$, $y_1=\frac{k}{2}$ and $y_2=k\cos\alpha$, then we obtain standard LMI problem

$$A_0+y_1A_1+y_2A_2 \succ 0$$

Lee
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  • Have you tried solving a semidefinite program with zero objective? – Rodrigo de Azevedo Sep 16 '21 at 06:15
  • yes, I have posted a question in "computational science" where I have tried to use SDP: https://scicomp.stackexchange.com/questions/40060/find-x-that-satisfy-i-aax-fracaa20-using-lmi-or-sdp-on-matlab – Lee Sep 16 '21 at 06:17
  • It is my mistake, I have changed transpose to conjugate transpose. Now $A_0,A_1$ and $A_2$ are all real. – Lee Sep 16 '21 at 07:18
  • I don't understand why you don't use SDP. – Rodrigo de Azevedo Sep 16 '21 at 07:31
  • as I have mentioned in the above comment, I have tried SDP, but couldn't solve the problem. – Lee Sep 16 '21 at 08:09
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    @Lee Use -(L0+x*L1) == hermitian_semidefinite(n) in your another post https://scicomp.stackexchange.com/questions/40060/find-x-that-satisfy-i-aax-fracaa2-prec0-using-lmi-or-sdp-on-ma. – River Li Sep 26 '21 at 00:57
  • @RiverLi thanks, I have changed it already. Forgot to update here – Lee Sep 26 '21 at 05:29
  • @Lee You can use the standard form of LMI in cvx/Matlab? By the way, did you change it in https://scicomp.stackexchange.com/questions/40060/find-x-that-satisfy-i-aax-fracaa2-prec0-using-lmi-or-sdp-on-ma? – River Li Sep 26 '21 at 05:53
  • @RiverLi I think we can use standard form of LMI in Matlab, recently I found some codes using lmivar, lmiterm, lmis and feasp functions – Lee Sep 26 '21 at 05:56

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