If I have a vector w and index it with, say, j then if w appears in another term in the equation do I use the same index i.e. j? Thanks
An example would be helpful. Usually is considered a function that maps a natural number into the th component of (or maps a sequence of vectors into the th vector, depending on the meaning of "index with "). Then, as in any function, the argument can be anything, e.g., 0, , , etc. But if you make a convention (even implicitly) that ranges over vector indices, it is better to keep using for this and not change it to, say, without reason.