δ

It hit me that $\delta(x)$ and $\delta_{ij}$ are the same thing, they both mean “peak”, and $x=i-j$ or $x=\vert i-j \vert$. That’s why they use the same symbol $\delta$.

• $\delta(x)$:

• $\delta_{ij}= \begin{cases} 1 &\mathrm{if } i=j \\ 0 &\mathrm{if } i \not = j \end{cases}$