Comment by yorwba

Its number of occurrences is 103,090. In the master's thesis identified as the original source https://cs.uwaterloo.ca/~smwatt/home/students/theses/CSo2005... the Unicode value of the operator occurring 103,090 times is given as 2061, and the thesis helpfully explains that

Unicode 2061, 2062 and 2063 are invisible operators. TeX does not have any of these invisible operators. These invisible operators result from the TEX to MathML conversion.

– 2061 – Function application

– 2062 – Invisible times

– 2063 – Invisible separator

And Wikipedia says that function application may be represented as

U+2061 FUNCTION APPLICATION (⁡, ⁡) — a contiguity operator indicating application of a function; that is an invisible zero width character intended to distinguish concatenation meaning function application from concatenation meaning multiplication. https://en.wikipedia.org/wiki/Function_application#Represent...

I'm not sure though how an automated conversion process would be able to distinguish between these.