Question

What does `[3, oo)` mean?

Answer 1

[3, ∞) represents the set of numbers between the range of 3 and infinity.

The square bracket means that 3 is included in this set of numbers, and the round bracket means that infinity is not included.

Because infinity is a concept of a number that can never be reached (you can always add one more), the round bracket means that this is the set of all numbers greater than or equal to 3.