I have a mathematical model, where the following variables are used:
- $x_i$,
- $x_{i,i}$ and
- $x_{i, j}$, $i \neq j$.
There is a need to express these variables with just one variable, using a "wildcard" character in the index of the variable that could denote any of the above cases.
For example, I could define $x_{i\_}$ to denote either $x_i$, $x_{i,i}$ or $x_{i,j}$.
I understand that there is not a standard way to do this, and it would be fine as long as I define it properly, but I would like to use a notation that would not cause any confusion. Traditionally, in computer programming, a wildcard character is either "*" or "%", but I believe these characters could cause confusion in the particular context. Any suggestions would be much appreciated.