In a mathematical model, I have a set $J$ of distinct jobs, and a constraint dealing with pairs of elements from the job set. Which of the following is the most accurate way to address two jobs $j$ and $j'$ from the set $J$?
$j,j' \in J$
$(j,j') \in J$
$j,j' \in J^2$
$(j,j') \in J^2$
In short, I want to know whether I need brackets, and the set should be $J$, or $J^2$?
** Perhaps a basic question, but I want to be accurate in my writing.