Question

What does the mathematical expression 9 ∉ T mean?

Answer 1

`9 !in T` means that `9` is not an element of the set `T`.

Explanation:

The symbol `!in` is used with a set `S` and a potential element `a` of `S` to mean

`a !in S`

if and only if `a` is not an element of `S`. Some people like to say that `a` "does not belong" to set `T`.

(compare to `in` which implies an element is in a set)

Thus `9 !in T` means that `9` is not an element of the set `T`