You notate the operation with * and $ (in mathup I use .* and .$ to make the operator invisible). Literal π is used both as a constant and as a function identifier depending on context so π(1 + n) can either be e.g. the (n+1)th prime or pi times one plus n. It is usually clear from context which one the author meant but if it is a problem explicit notation could help for sure.

I have also heard that the invisible operators help with accessibility. I haven’t tested this on my screen reader, but I suppose <mi>π</mi><mo>&af;</mo><!-->...<--> would read something like “Pi of ...” whereas <mi>π</mi><mo>&InvisibleTimes;</mo><!-->...<--> would read something like “Pi times ...” all the while <mi>π</mi><mrow><mo>(</mo> would just read “Pi [open parenthesis] ...”.