sum_(i=1)^n
How about using some symmetry instead, e.g.
sum(i=1)(n) "Means: sum from i=1 to n"The caret is used to indicate the upper limit for the same reason some programming languages use it as the exponentiation operator (other programming languages may use something else, like **, neither is normally how exponentiation is “normally”, outside of programming and its historic limitation to ASCII characters, indicated), because its upward-pointing character is a considered a way of suggesting that the following number should be thought of as presented raised from the normal baseline, which is (in somewhat different ways) true of both exponents and upper limits in summation (this is the mirroelr image of why _ is used for the lower limit.)