N: non-negative integer set (including 0, excluding negative numbers and fractions) Z: positive integers, zero and negative integers are collectively referred to as integers (including 0 and negative integers, excluding fractions) Q: rational numbers are collectively referred to as integers and fractions (including 0, negative numbers and fractions) R: real numbers include all rational numbers and irrational numbers, and are collectively referred to as rational numbers and irrational numbers. (including 0, negative numbers and fractions)