Sequence is a function with positive integer set (or its finite set) as its domain, and it is an ordered number. Every number in a series is called an item in this series.

The number ranked first is called 1 item of this series (usually also called the first item), the number ranked second is called the second item of this series, and so on, and the number ranked n is called the n item of this series, usually represented by an.

Integer is a set of positive integers, zero and negative integers. All integers form an integer set, which is a number ring. In the integer system, zero and positive integers are collectively called natural numbers. -1, -2, -3, …, -n, …(n is a non-zero natural number) are negative integers. Positive integers, zero integers and negative integers form an integer system. Integers exclude decimals and fractions.

The relationship between structural mathematics and unstructured mathematics is manifested in "* * * nature" and "bifurcation". Up to now, the progress of mathematical construction methods has been directly obtained by standard unstructured mathematical ideas. Therefore, people often have an illusion that structural mathematics "parasitizes" unstructured mathematics and develops.

In fact, structural mathematics can provide more natural and simpler proofs for some theorems than unstructured mathematics, and may even draw some new theorems of unstructured mathematics. So the relationship between these two kinds of mathematics is complementary.