[runarberg]

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] ...”.

[eviks]

Good point about primes, not enough symbols for uniqueness, so just π isn't enough! Though using two symbols ( .* ) in place of nothing is too dirty, better use an invisible and color highlight the spacing between π and ( to indicate to the plain text reader the difference.

Win screenreaders understands 2.3 (pronounced two three) vs 23 (twenty three), though it doesn't pronounce the operator, neither in a.2 vs a2 (though pronounced differently). But this could be fixable, so also good point about accessibility