A natural number is any positive integer: . The set of natural numbers, denoted , is a subset of the set of integers, . Some texts use to denote the set of positive integers (sometimes called counting numbers in elementary contexts), while others use if to represent the set of nonnegative integers (sometimes called whole numbers). In particular, usually includes zero in the contexts of set theory and algebra, but usually not in the contexts of number theory. When there is risk of confusion, mathematicians often resort to less ambiguous notations, such as and for the set of non-negative integers, and and for the set of positive integers.
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