Update src/7_tight_binding_model_sol.md

src/7_tight_binding_model_sol.md

@@ -35,8 +35,8 @@ Group velocity is given as $v=\hbar^{-1}\frac{\partial E}{\partial k}$ with $E=\
 
 ```python
 pyplot.subplot(1,2,1)
-k = np.linspace(-pi/2, pi/2, 300)
-pyplot.plot(k, np.cos(k));
+k = np.linspace(-pi/2+0.01, pi/2-0.01, 300)
+pyplot.plot(k, np.sin(k)/(np.sqrt(1-np.cos(k)));
 pyplot.xlabel('$k$'); pyplot.ylabel('$v(k)$');
 pyplot.xticks([-pi/2, 0, pi/2], [r'$-\pi/2$', 0, r'$\pi/2$']);
 pyplot.yticks([0, 1], [0, r'$2\sqrt{\frac{\kappa}{m}}$']);