# Four

Four
4th
presentation
Roman IV
dual 100
Octal 4th
Duodecimal 4th
Morse code · · · · -
Arabic ٤
Chinese
Mathematical properties
sign positive
parity straight
Factorization ${\ displaystyle 2 ^ {2}}$
Divider 1, 2, 4
A continental European handwritten four and a notation common in the English-speaking world
Four in flag alphabet
Half eight as four in round spelling in the year 1497
Half eight as four in square spelling in the year 1488

The four (4) is the natural number between three and five . It's even and a square number .

## mathematics

The four is straight. It is the first composite number and thus the first non- prime number after the one .

The peculiarity of the four is that both and and thus apply. This can even be generalized using the arrow notation. It applies to any natural : ${\ displaystyle 2 + 2 = 4}$${\ displaystyle 2 \ cdot 2 = 4}$${\ displaystyle 2 ^ {2} = 4}$${\ displaystyle n}$

${\ displaystyle 4 = 2 \ \ underbrace {\ uparrow \ \ uparrow \ cdots \ uparrow} _ {n \ \ mathrm {times}} \ 2}$.

The operator in the famous Pythagorean triple can be "scooped" from the (degenerate) triple using the four operators required to calculate all primitive Pythagorean triples . ${\ displaystyle 4}$${\ displaystyle (3,4,5)}$${\ displaystyle 0}$${\ displaystyle (1,0,1)}$

Four points span a square , a surface with four sides, in the plane . It is the simplest figure that can be deformed while retaining its side lengths, for example from a rectangle to a parallelogram.

A maximum of four points can be equidistant in space, i.e. H. at the same distance from each other. These then form a tetrahedron , a body with four equal triangular sides.

The four-color theorem states that four colors are sufficient to color all areas on a map in such a way that areas of the same color (along borders) do not collide anywhere, whereas three colors are not sufficient for this.

Another peculiarity of the four is the impossibility of solving an algebraic equation of a higher degree than four with the help of extracting the roots and simple arithmetic operations. The mathematician Niels Henrik Abel published his proof of this in 1826 .

Four is a Størmer number .

## Spellings

The number has different glyphs depending on the age and language :

The spelling in the Roman numerals is IV, the Arabic ٤. In Urdu (India, Pakistan) it is ۴, in Devanagari (India) it is ४ and in Hebrew it is ד. In the European Middle Ages , the spelling was common as half an eight.

In Germany, the number 4 is handwritten in two lines according to the numerical spelling of the Latin original script : a right angle with a longer vertical line. This notation corresponds to the Austrian school script (both versions from 1969 and 1995) and the Swiss Schnürlischrift . In the English-speaking culture and in areas influenced by it, a 4 is drawn mostly closed.

The four is often represented as IIII on the dials of clocks with Roman writing.

## Linguistic

The Latin word for four is quattuor , the associated ordinal number quartus (the fourth) and the adjective quadrus (square) found their way into German with terms such as quarter , fourth , quartile , quartet or square .

The Greek prefix for four τετρα- (tetra-) was used for foreign words such as tetralogy , tetrarchy , tetrahedron or tetrachord .

## Natural sciences

### biology

The information of the genome of all earthly life stored in the deoxyribonucleic acid is encoded in triplet sequences, which consist of four different basic units: the nucleobases adenine (A), thymine (T), guanine (G) and cytosine (C).

The land vertebrates (tetrapods) , the quadruped land vertebrates, basically each have four extremities, the elementary importance of which is locomotion. Individual tetrapods can have all or some of the extremities regressed or remodeled, such as snakes that no longer have locomotion-capable extremities or humans in whom the front extremities are no longer suitable for locomotion. The tetrapods include amphibians (amphibians), reptiles (reptiles), mammals and birds.

### chemistry

The number 4 is the ordinal number of the alkaline earth metal beryllium in the periodic table .

The carbon atom has the property of four covalent bonds to establish in tetrahedral angles and thereby can the extremely hard diamond and a wide variety of organic molecules of chains with ramifications, chirality , double bonds, to form to ring systems.

## medicine

In orthopedics , the four-digit symbol is a clinical test to test the functionality of the hip joint in humans. When lying down, one leg is bent to a four across the body axis.

## Culture, literature, music, society and intellectual life

The Greek natural philosophers saw four elements (fire, water, earth, air) as the basic components of all being: the four-element doctrine is still effective today. Since there are four cardinal points, seasons and temperaments in addition to the four elements , the four was considered the number of the earthly in the Middle Ages, in contrast to the three , the number of God and the Trinity . From the arithmetical connections of these numbers three and four there are other numbers that are sacred in Christianity: seven and twelve . From this Christian doctrine of numbers, the conception arises for music that the 3/4 time is considered perfect - previously represented by a circle, while the 4/4 time is considered the imperfect, earthly one, which is still in the today musical notation is indicated by a semicircle, a character similar to the capital C.

In the number symbolism it stands for the cross, among other things. It crosses in its representation and the cross has four corners. It therefore stands for death and suffering in general. In the Chinese number symbolism and also in Korea and Japan, the four is considered an unlucky number , as its sound ( Chinese   , Pinyin ) is similar to the Chinese word for death ( Chinese   , Pinyin ) see: tetraphobia .

## Sports

The number 4 on the back is the number of the pre-stopper / central defender in football.