Update index.md

Author: mtian8

docs/index.md

@@ -232,14 +232,21 @@ We denote $\{\mathbf{a}_i\}$ are the vectors from a B site to its three nearest-
 
 $$
 {\mathbf{a}_1} = (0,a),
+$$
 
-{\mathbf{a}_2} = (\sqrt 3 a/2, - a/2),
+$$
+{\mathbf{a}_2} = (\sqrt 3 a/2, - a/2),{\mathbf{a}_3} = ( - \sqrt 3 a/2, - a/2)
+$$
 
-{\mathbf{a}_3} = ( - \sqrt 3 a/2, - a/2)
+$$
 {\mathbf{b}_1} = {\mathbf{a}_2} - {\mathbf{a}_3} = (\sqrt 3 a,0),
+$$
 
+$$
 {\mathbf{b}_2} = {\mathbf{a}_3} - {\mathbf{a}_1} = ( - \sqrt 3 a/2, - 3a/2),
+$$
 
+$$
 {\mathbf{b}_3} = {\mathbf{a}_1} - {\mathbf{a}_2} = ( - \sqrt 3 a/2,3a/2)
 $$