What would I use instead of $\displaystyle \sum$ if I just wanted to sum odd values for some number n through k?
I think you mean start from n and end at k by 2's. You can do this a few ways.
$\displaystyle \sum_{n=1,3,5...}^{k} a_n$ works but looks bad.
If you want to sum over all odds you can write $\displaystyle \sum_{odds}$
If you want to skip terms in a sequence you can write
$\displaystyle \sum_{n}^{k} a_{2n}$
I don't know exactly what you meant but those are some of my guesses.