# How is this a linear matrix inequality?

In example 3.4 of Stephen Boyd & Lieven Vandenberghe's Convex Optimization, it is mentioned that the last condition of

$$\text{epi} = \left\{ (x,Y,t) \mid Y \succ 0, x^T Y^{-1} x \leq t \right\}$$

is a linear matrix inequality (LMI) in $$(x,Y,t)$$. However the linear matrix inequality is written as (in Eq. 2.11 of same book)

$$A(x) = x_1 A_1 + x_2 A_2 + \cdots + x_n A_n \preceq B$$

where $$A_i$$ and $$B$$ are symmetric matrices. How to show that $$x^TY^{-1}x\leq t$$ is a linear inequality in $$(x,Y,t)$$? Any help in this regard will be much appreciated. Thanks in advance.

Using the Schur complement [JG'10], if $\rm Y \succ O$ then $t - \rm x ^\top Y^{-1} x \geq 0$ is equivalent to the LMI
$$\begin{bmatrix} \mathrm Y & \mathrm x\\ \mathrm x^\top & t\end{bmatrix} \succeq \mathrm O$$
• If $\rm Y$ is $2 \times 2$, then \begin{align} \begin{bmatrix} y_{11} & y_{12} & x_1\\ y_{12} & y_{22} & x_2\\ x_1 & x_2 & t\end{bmatrix} &= y_{11} \begin{bmatrix} 1 & 0 & 0\\ 0 & 0 & 0\\ 0 & 0 & 0\end{bmatrix} + y_{12} \begin{bmatrix} 0 & 1 & 0\\ 1 & 0 & 0\\ 0 & 0 & 0\end{bmatrix} + y_{22} \begin{bmatrix} 0 & 0 & 0\\ 0 & 1 & 0\\ 0 & 0 & 0\end{bmatrix} +\\\\\ &\,\, + x_{1} \begin{bmatrix} 0 & 0 & 1\\ 0 & 0 & 0\\ 1 & 0 & 0\end{bmatrix} + x_{2} \begin{bmatrix} 0 & 0 & 0\\ 0 & 0 & 1\\ 0 & 1 & 0\end{bmatrix} + t \begin{bmatrix} 0 & 0 & 0\\ 0 & 0 & 0\\ 0 & 0 & 1\end{bmatrix}\end{align} May 31 '18 at 9:48