1. Mathematical n is a set of natural numbers in a set, a set of natural numbers is a set of all non-negative integers, and there are infinitely many natural numbers.

2. Natural numbers are signs of the same characteristics of all equivalent finite sets.

3. Integers include natural numbers, so natural numbers must be integers and non-negative integers.

Natural numbers play a great role in daily life, and people use them widely.

5. Natural numbers are the earliest numbers in human history and are widely used in counting and measurement.

People often use natural numbers to mark or classify things, such as bus routes, house numbers and postal codes of cities.