N in mathematics represents a set of natural numbers in a set. N+ represents a set of positive integers, Z represents a set of integers, Q represents a set of rational numbers, and R represents a set of real numbers. A non-negative integer set is a specific set, which refers to the set of all natural numbers, and is commonly represented by the symbol n. Non-negative integers include positive integers and zeros.

A set of nonnegative integers is a countable set. In the non-negative integer set, there is a minimum natural number 0; After dividing by zero in n, the number set composed of other natural numbers is called positive integer set, which is usually represented by the symbol N+ or n, and 1 is the smallest element in n; There is no maximum natural number in n and N+; Are infinite sets.

Characteristics of sets

A sure thing

Given a set, any element, whether it belongs to the set or not, must be one of them, and no ambiguity is allowed.

Mutual anisotropy

Any two elements in a collection are considered different, that is, each element can only appear once. Sometimes it is necessary to describe the situation where the same element appears many times. You can use multiset, where elements are allowed to appear multiple times.

randomness

In a set, the state of each element is the same and the elements are out of order. You can define an order relation on the set. After defining the order relation, you can sort the elements according to the order relation. But as far as the characteristics of the set itself are concerned, there is no necessary order between elements.

The four functions of natural numbers include counting, measuring, marking and sorting.

count

Counting is a mathematical behavior of repeatedly adding (or subtracting) 1, which is usually used to calculate how many objects there are or to place the required number of objects (counting the first object from the beginning and corresponding the remaining objects to natural numbers starting from two).

I have seven apples here. The teacher brought 30 new textbooks.

measure

Measurement is to describe the observed phenomena with data according to certain laws, that is, to describe things quantitatively. Measurement is the quantization process of non-quantized objects.

This road is 2000 meters long. Brother height 180 cm.

grade

Labeling is to label items and articles with serial numbers (in the order of 1, 2, 3, 4, 5, 6, ...), which makes articles and articles more clear.

For example: 1 room, room 2, room 3. Paragraphs 1, 2 and 3.

classify

Sorting is to adjust a group of "unordered" record sequences into "ordered" record sequences.

This is the second largest city in China. Mount Everest is the highest mountain in the world.