Update asvoigt.md

docs/api/functions/asvoigt.md

@@ -10,8 +10,52 @@ grand_parent: API Reference
 
 Converts a `Tensor2` or `Tensor4` data type to a reduced Voigt-storage `Tensor2s` or `Tensor4s`.
 
-- `asvoigt(T)`
-
 Aliases:
 - `voigt(T)`
-- `symstore(T)`
+- `symstore(T)`
+
+$$
+\begin{align}
+\boldsymbol{C} = \begin{bmatrix}
+    C_{11} & C_{12} & C_{13} \\
+    C_{12} & C_{22} & C_{23} \\
+    C_{13} & C_{23} & C_{33}
+\end{bmatrix} \longrightarrow
+\begin{bmatrix}
+    C_{11} \\ C_{22} \\ C_{33} \\ C_{12} \\ C_{23} \\ C_{13}
+\end{bmatrix}
+\end{align}
+$$
+
+$$
+\begin{align}
+\mathbb{A} &= \begin{bmatrix}
+    A_{1111} & A_{1112} & A_{1113} &
+    A_{1121} & A_{1122} & A_{1123} &
+    A_{1131} & A_{1132} & A_{1133} \\
+    A_{1211} & A_{1212} & A_{1213} &
+    A_{1221} & A_{1222} & A_{1223} &
+    A_{1231} & A_{1232} & A_{1233} \\
+    \dots & \dots & \dots & \dots & \dots & \dots & \dots & \dots & \dots \\
+    A_{3111} & A_{3112} & A_{3113} &
+    A_{3121} & A_{3122} & A_{3123} &
+    A_{3131} & A_{3132} & A_{3133}
+\end{bmatrix} \nonumber \\
+&\longrightarrow
+\begin{bmatrix}
+    A_{1111} & A_{1122} & A_{1133} & A_{1112} & A_{1123} & A_{1113} \\
+    A_{2211} & A_{2222} & A_{2233} & A_{2212} & A_{2223} & A_{2213} \\
+     \dots   &  \dots   &  \dots   &  \dots   &  \dots   &  \dots   \\
+    A_{1311} & A_{1322} & A_{1333} & A_{1312} & A_{1323} & A_{1313}
+\end{bmatrix}
+\end{align}
+$$
+
+### Example
+
+```fortran
+type(Tensor2) :: T
+type(Tensor2s) :: U
+
+U = asvoigt(T)
+```