It seems like the equation processing gets messed late in section 4 - $$ \mathrm{DFT}[k] = \sum_{n=0}^{N-1} \mathrm{x}[n] \cdot (cos(\varphi) - sin(\varphi)i) \\ where \quad \varphi = k \frac{n}{N} 2\pi $$

https://jackschaedler.github.io/circles-sines-signals/dft_wa...

there are a few other examples in section 5

Aside from that, this is really great.