The Null set or Empty set

There are some sets that do not contain any element at all.

For example, the set of months with 32 days.

We call a set with no elements the null or empty set.

It is represented by the symbol {} or Ø

The empty set is always a null set. More generally, any countable union of null sets is null. Any measurable subset of a null set is itself a null set.

More generally, any countable union of null sets is null.

Any measurable subset of a null set is itself a null set.

So, from the definition, null set of U is U itself.