LAtex fix

docs/background.md

@@ -44,18 +44,11 @@ $S_{\rm vN}(i)$ is the von Neumann entropy of the subsystem
 $[i+1,\cdots,N]$.
 
 
-What about mixed states? They can be represented using so-called matrix-product operators (MPO):
-$\rho=\sum_{a_1,a_2,\cdots,a_N}{\rm Tr}\left[M^{(a_1)}_1 M^{(a_2)}_2\cdots M^{(a_N)}_N\right]\sigma^a_1 \otimes \sigma^a_2 \otimes \cdots \sigma^a_N$
-where each $a_i$ can take four values
-$\in\{1,x,y,z\}$,
-$\sigma^{a_i}$ is a Pauli matrix or the identity acting on qubit
-$i$,
-and we have associated  four matrices
-$M_i^{(1)},
-$M_i^{(x)}$,
-$M_i^{(y)}$ and
-$M_i^{(z)}$ to each qubit.
-
+What about mixed states? They can be represented using so-called matrix-product operators (MPO),
+  ```math
+\rho=\sum_{a_1,a_2,\cdots,a_N}{\rm Tr}\left[M^{(a_1)}_1 M^{(a_2)}_2\cdots M^{(a_N)}_N\right]\sigma^a_1 \otimes \sigma^a_2 \otimes \cdots \sigma^a_N
+```
+where each $a\_i$ can take four values $\in\{1,x,y,z\}$, $\sigma^{a_i}$ is a Pauli matrix or the identity acting on qubit $i$, and we have associated four matrices $M_i^{(1)}, $M_i^{(x)}$, $M_i^{(y)}$ and $M_i^{(z)}$ to each qubit.
 
 
 ### Bond dimension and entanglement