The S-exp can be shortened in anything calling itself a Lisp, using the unary reciprocal: (+ (/ 2) 5).

Math papers are full of obscure notations; they should standardize on s-exps. Then just a straightforward dictionary of symbols would be needed to look up a notation.

Math notation is at least 2D: it makes good use of spatial structures to reduce ambiguity. For instance, instead of inventing some crazy ternary operator, mathematicians will just arrange three arguments spatially around a symbol. The space subdivides recursively, so elements end up being positionally determined, somewhat like what S-exps do in 1D.

That being said, a lot of mathematical notation is pretty close to lisp syntax, basically anything that works like sigma.