I'm just reading through some vector calculus notes and it begins to assume knowledge of the notation $\delta_{ij}$ and $\epsilon_{ijk}$ in reference to tensors, but I am not familiar with this. It uses them as if they are constant and well-defined.
If someone could just explain this I would appreciate it. Thank you.
see here: https://en.wikipedia.org/wiki/Kronecker_delta
and here: https://en.wikipedia.org/wiki/Levi-Civita_symbol
these are well known usefull shorthands.