Bloch theorem as neat as it should be

I don't think anyone would disagree if i say that Bloch theorem is "A" of the alphabet for solid state physics today, especially with all the planewave approaches becoming the main if not only tools for solid state simulations, even when crystal symmetry is discarded, as in surfaces etc.

So here is the cutest explanation of Bloch theorem I have seen so far, thank you for writing it down!

the pdf is here