Update index.md

Author: mtian8

docs/index.md

@@ -223,7 +223,7 @@ import cmath
 from math import pi, sin, cos, sqrt
 ```
 ### Subproblems
-**1.1 Write a Haldane model Hamiltonian on a hexagonal lattice, given the following parameters: wavevector components $k_x$ and $k_y$ (momentum) in the x and y directions, lattice spacing $a$, nearest-neighbor coupling constant $t_1$, next-nearest-neighbor coupling constant $t_2$, phase $\phi$ for the next-nearest-neighbor hopping, and the on-site energy $m$.**
+**1.1 Write a Haldane model Hamiltonian on a hexagonal lattice, given the following parameters: wavevector components $$k_x$$ and $k_y$ (momentum) in the x and y directions, lattice spacing $a$, nearest-neighbor coupling constant $t_1$, next-nearest-neighbor coupling constant $t_2$, phase $\phi$ for the next-nearest-neighbor hopping, and the on-site energy $m$.**
 
 **_Scientists Annotated Background:_**
 Source: Haldane, F. D. M. (1988). Model for a quantum Hall effect without Landau levels: Condensed-matter realization of the" parity anomaly". Physical review letters, 61(18).