Function application without parentheses
This is an example of juxtaposition means combine in the obvious way.
This is most usually done with the trigonometric functions and logarithms, e.g.
\[ \sin \theta, \ln x \]
Deyan Ginev searched for occurrences of
f x in arXiv papers and found about 7,000 papers containing that pattern, containing a wide range of exotic uses. In a handful of which, "f x" was indeed a function application.