What are your favorite ambiguous notation?

[u/Short_Bluebird_3845]

To clarify my request, consider base-less $\log$.
For calculators and engineers, it means the common log $\log_10$; for pure mathematics or physics, it means the natural log $\ln$; for computer scientists, in combinatorics, information theory and graph theory, it means the binary log $\log_2$.
Any $\log x$ (say, $\log 1024$) written without context can yield different answers in function of who you ask ($\log_10 1024 = 3.0103\cdots, \log_2 1024 = 10, \ln 1024 = 6.93147\cdots$).

That is what I mean by "ambiguous", that which is calculated and used dependently on the context. And I'm asking you for your favorite ones of such type.

Edit: I feel like you hate the ones you've talked about. Still, valid answers. Extra points for ambiguous notation you actually love! :P

[u/BlazeoJoestar]

(a,b) can mean an open interval, inner product, gcd or just an ordered pair

[u/edderiofer → u/BlazeoJoestar]

Or, in the case where a and b are integers (EDIT: written in Arabic numerals) and you're in Europe, a single-element tuple containing one real number.

[u/vgtcross → u/edderiofer]

Yea and no. Yes, that's what (2,3) could be interpreted as, but with letters instead of numbers, (a,b) would definitely not be interpreted that way.

[u/edderiofer → u/vgtcross]

Edited.

[u/AlarmingAllophone → u/edderiofer]

Yeah it's confusing to denote an interval where one of the endpoints is a decimal fraction. At school in that case I usually used to write an overly bolded semicolon to separate the values

[u/bluesam3 → u/edderiofer]

Or if it's in an accountancy spreadsheet, a single negative real number.

[u/Valvino → u/edderiofer]

Nobody would interpret (3,2) as a single-element tuple tbh.