Function application without parentheses

It's sometimes OK to write a function name followed by its argument, without any parentheses. Because there's there is no function application symbol, this can look like a multiplication.

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.

\[ \sin \theta, \ln x \]

So is it only OK to omit the parentheses for well-known functions? Or is it for any function whose name is longer than one letter? Christian Lawson-Perfect asked, and got mixed responses.

Deyan Ginev searched for occurrences of  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.