# Change of Index Variable of Summation

$\displaystyle \sum_{R \left({i}\right)} a_i = \sum_{R \left({j}\right)} a_j$
where $\displaystyle \sum_{R \left({i}\right)} a_i$ denotes the summation over $a_i$ for all $i$ that satisfy the propositional function $R \left({i}\right)$.