There seems to be an issue with the math display in question 3 solutions of Band structures in 2D (Band structures in 2D - Open Solid State Notes).
I reloaded the page multiple time but the problem persists, so I believe it’s an issue within the book itself. My excuses if it’s just my laptop after all.
I also ran it in latex to check if I could find the error but except disliking the nesting of align within equation it managed fine, so I’m unsure what the issue is.
This is what it displays for me:
I think maybe having eqution and align in this way could be the issue. This worked for me:
Using only align (without dollar signs) and using \begin{equation} with our without dollar signs and with our without align just caused it not to display it properly.
(and if I may be very nitpicky and causing you to open the files already anyways there’s a small spelling error in chapter 11 as well, the second if in this setence:)
sorry ^^;
Thank you for your feedback, I applied the changes in the repository. Hopefully they will be implemented soon 
