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Natural numbers.
1.18 is a number and number do not contain any sets (of any kind).
-10 belongs to the set of all integers denoted by Z.
173 belongs to the set of positive natural numbers and it also belongs to the set of prime numbers.
Infinity is not a number. There are different classes of infinity: The sets of natural numbers, integers, rational numbers all belong to the smallest class, with a cardinality of Aleph-null. The sets of irrational numbers and real numbers belong to the next higher level of infinity, with cardinality Aleph-One. Infinity can give rise to a very large number of apparent paradoxes - infinitely many of them?