I tried to make a variable named “T_ransparency” (note the consecutive letters “ans” in the subscript).
I get the error “Subscripts may only contain letters and digits. ‘operatorname{ans}’ is not allowed.” In the subscript, the letters “ans” are upright, the way they are when you type them at the top level.
Oddly, this exact problem seems to only happen with “ans”; putting “sin”, “total”, “quantile”, or anything else works the way I expected. However, putting in letters that automatically get transformed into symbols causes the problem: it’s difficult to put “sqrt”, “int”, etc. in subscripts.
With a bit of tinkering, I found a workaround: type, for example, “anQs”, then go back and delete the Q. The text stays italicized and the variable name is valid.
However, although there is an easy workaround, it seems to me that, for the sake of consistency and clarity, “ans” and “sin” should behave the same way. (Perhaps also “sqrt”, “int”, etc should stay as regular characters in subscripts too.)
Here’s a link demonstrating all of the above: "ans" bug