Number Four
Introduction
The number four (4) is a natural number that follows the number three and precedes the number five. It is an integer that is considered even, as it is divisible by two without leaving a remainder. The number four is significant in various fields, including mathematics, science, culture, and religion.
Mathematics
In mathematics, four is the smallest composite number, its proper divisors being one and two. The number four is also the second square number, as it can be expressed as 2^2. It is the only square one more than a prime number. A four-sided plane figure is a quadrilateral, which can be divided into two triangles. In addition, four is the maximum number of dimensions of a real division algebra (read more), as stated in Hurwitz's theorem.
Number Theory
In number theory, four is a highly composite number. It is the smallest number of colors sufficient to color all planar maps, according to the four color theorem. It is the smallest number that is equal to the sum of the digits of its cube (4^3 = 64, and 6 + 4 = 10 = 2 * 4). The smallest non-cyclic group has four elements, it is the Klein four-group. Four is also the order of the smallest non-trivial groups and is the only integer n for which the (non-trivial) alternating group A_n is not simple.
Geometry
In geometry, four is the number of dimensions in Euclidean space. It is also the number of points needed to specify a plane. A tetrahedron, which is the simplest form of a polyhedron, has four faces. A square, which is a regular quadrilateral, has four sides of equal length and four right angles.
Science
In science, four is the atomic number of beryllium. It is also the number of nucleobase types in DNA and RNA – adenine, guanine, cytosine, and thymine (uracil in RNA). In addition, it is the number of basic states of matter: solid, liquid, gas, and plasma.
Culture and Religion
In many cultures and religions, the number four is of great significance. For example, in Buddhism, there are Four Noble Truths. In Christianity, there are four Gospels in the New Testament. In many East Asian cultures, the number four is considered unlucky because it is a homophone for the word "death".