Science Fair Project Encyclopedia
- This article discusses the number Four. Four is also the name of a French commune, Four, Isère .
|Cardinal|| 4 |
|Ordinal|| 4th |
|Divisors||1, 2, 4|
|Roman numeral||IV or IIII|
|Unicode representation of Roman numeral||Ⅳ, ⅳ|
|prefixes||tetra- (from Greek)|
Evolution of the glyph
Representing 1, 2 and 3 in as many lines as the number represents worked well enough, but with 4 writing four lines began to appear tiresome. The Brahmin Indians simplified 4 by joining its four lines into a cross that looks like our modern plus sign. The Sunga and other Indians would add a horizontal line on top of the numeral, and the Kshatrapa and Pallava evolved the numeral to a point where speed of writing was at best a secondary concern. The Arabs didn't have time for cursive fancy: their 4 still had the early concept of the cross, but for the sake of efficiency, was made in one stroke by connecting the "western" end to the "northern" end; the "eastern" end was finished off with a curve. The Europeans dropped off the finishing curve and gradually made the numeral less cursive, ending up with a glyph that could have evolved in a much simpler way than the convoluted route it actually took: by simply taking the Brahmin cross and adding a line to connect its "western" and "northern" ends.
Four is the smallest composite number, its proper divisors being and 2. Four is also a highly composite number. The next highly composite number is 6. Four is the second square number, the second centered triangular number.
Four is the only composite number that is equal to the sum of its prime factors. As a consequence of this, it is the smallest Smith number.
It is also a Motzkin number.
In addition, . Continuing the pattern in Knuth's up-arrow notation, , and so on, for any number of up arrows.
The four-color theorem states that a planar graph (or, equivalently, a flat map of two-dimensional regions such as countries) can be colored using four colors, so that adjacent vertices (or regions) are always different colors. Three colors are not, in general, sufficient to guarantee this.
Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four square numbers. Three are not always sufficient; 7 for instance cannot be written as the sum of three squares.
Four is the number of n-Queens Problem solutions for n = 6.
The Cartesian coordinate system for a plane has four quadrants.
Four is the first positive non-Fibonacci number.
Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. 4x = y2 - z2.
- Valency of carbon (that is basis of life on the Earth) is four. Thanks to its tetrahedral crystal bond structure diamond (one of the natural allotropes of carbon) is the hardest known naturally occurring material.
- The atomic number of beryllium
The fourth finger of a human (on the left hand - ring-finger) is moved when the little finger moves.
There are four human blood groups.
People have four canines, four incisor and four wisdom teeth.
Four of Jupiter's moons are readily visible from Earth.
Most vehicles have four wheels.
Most furniture has four legs - tables, chairs, etc.
Four horses (quadriga) is the maximal number of horses in one row for carriage.
Wide use of rectangles (with four angles and four sides) because they have effective form and capability for close adjacency to each other (houses, rooms, tables, bricks, sheets of paper, screens, film frames).
In other fields
In technologically adept circles, the number four can be used as an obscenity, replacing the common word for fornication. This usage derives from the fact that when counting in binary on ones fingers, 4, whose binary representation is 001002 is represented by raising the middle finger of one hand, yielding a vulgar gesture. A stronger variation uses the number 132, whose binary representation 0010000100 replicates this gesture on both hands.
Four is the only number that, when spelled out in English, has the number of letters that it names.
Four seasons: winter, spring, summer and autumn.
Four parts of a day: night, morning, afternoon, evening.
Four is the figurative number of corners that the world has: North, South, West, East.
Four (四, formal writing: 肆, pinyin si4) in considered an unlucky number in Chinese and Japanese cultures because it sounds like the word "death" (死 pinyin si3). Due to that, many numbered product lines skip the "four": e.g. Nokia cell phones (there is no series beginning with a 4), Palm PDAs, the Leisure Suit Larry games, etc. Some elevators even skip floor 4, particularly in heavily Asian areas.
In the NATO phonetic alphabet, the digit 4 is called "fower".
- Gang of Four
- Four Daughters
- Four Noes and One Without
- Four Modernizations
- The bands The Four Tops and Four Non Blondes
- Led Zeppelin's fourth album, untitled but commonly referred to as Four Symbols or Led Zeppelin IV. It contains a song called Four Sticks
- Four great ancient capitals of China
- Four corner method
- Four-character idiom
- Four cardinal principles
- Four Books
- Four horsemen of the Apocalypse
- The number of strokes in the cycle of a four-stroke engine
- the number of musicians in a quartet
- The number of strings on a violin, a viola, a cello and a double bass
- The number of fingers that people have in many cartoons
- The number of suits of playing cards
- The number of teeth of most forks
- The number of nations of the United Kingdom: Wales, Scotland, Northern Ireland and England
- The number of Noble Truths in the Buddhist faith
- In the expression "fourth estate", referring to journalists.
- The designation of Interstate 4, a freeway in Florida
- Four, an album by the Blues Traveler
- The years: A.D. 4, 4 B.C., or 1904
- In rugby union, The number of the lock forward , who usually jumps at number 2 in the line-out.
- In baseball, four represents the second baseman's position.
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