Depending on the complexity, there’s also abbrev-mode: https://www.gnu.org/software/emacs/manual/html_node/emacs/Abbrevs.html

I wish this was exaggerated, but it isn’t at all. Every time I try to learn Haskell, I end up in some tutorial: “You know how you sometimes need to represent eigenvectors in an n-dimensional plane with isotonically theoretical pulsarfunctions? Haskell types make that easy!”

For what it’s worth, you can replace xi with just s or c