I am quite wondered how someone writes the permutation/combination as the part of a formula in the manuscript?
For example, let $x$ be a set of values, $x=\{x_1,x_2,...,x_n\}$. Then if someone want to write the $\sum (\text{combination of two}) +\sum(\text{combination of three})+...$. How can they do it? Just to add than I can use $\sum_{i=1}^{n-1}\sum_{j>i}^{n} x_i+x_j$ but imagine that you want to write $\sum\text{(combination of 10 out of n)}$!