[pfortuny]

Yes, the notation seems wrong but it is because O(...) is a set, not a function. The functions is what goes inside.

So it should be f€O(...) instead of f=...

(don't know how to write the "belongs" symbol on iOS).

[singhrac]

Here you go: ∈ (so f ∈ (...))

[pfortuny]

That helps but how do I learn it?