Evolution of the Arabic digit
Mathematics
Divisibility rule
A natural number is divisible by 3 if the sum of its digits in base 10 is also divisible by 3. This known as the divisibility rule of 3. Because of this, the reverse of any number that is divisible by three (or indeed, any permutation of its digits) is also divisible by three. This divisibility rule works in any positional numeral system whose base divided by three leaves a remainder of one (bases 4, 7, 10, etc.).Properties
3 is the second smallest prime number and the first odd prime number. 3 is a twin prime with 5, and a cousin prime with 7. A triangle is made of three sides. It is the smallest non-self-intersecting polygon and the only polygon not to have proper diagonals. When doing quick estimates, 3 is a rough approximation of , 3.1415..., and a very rough approximation of ''e'', 2.71828... 3 is the first Mersenne prime. 3 is also the first of five known Fermat primes. It is the second Fibonacci prime (and the second Lucas prime), the second Sophie Germain prime, and the second factorial prime. 3 is the second and only prime triangular number, and Gauss proved that every integer is the sum of at most 3 triangular numbers. Three is the only prime which is one less than a perfect square. Any other number which is − 1 for some integer is not prime, since it is ( − 1)( + 1). This is true for 3 as well (with = 2), but in this case the smaller factor is 1. If is greater than 2, both − 1 and + 1 are greater than 1 so their product is not prime.Numeral systems
There is some evidence to suggest that early man may have used counting systems which consisted of "One, Two, Three" and thereafter "Many" to describe counting limits. Early peoples had a word to describe the quantities of one, two, and three but any quantity beyond was simply denoted as "Many". This is most likely based on the prevalence of this phenomenon among people in such disparate regions as the deep Amazon and Borneo jungles, where western civilization's explorers have historical records of their first encounters with these indigenous people.List of basic calculations
Engineering
*The triangle, a polygon with three edges and three vertices, is the most stable physical shape. For this reason it is widely utilized in construction, engineering and design.Pseudoscience
*Three is the symbolic representation for Mu, Augustus Le Plongeon's and James Churchward's lost continent.Religion and beliefs
As a lucky or unlucky number
Three (, formal writing: , pinyin ''sān'', Cantonese: ''saam''1) is considered a good number in Chinese culture because it sounds like the word "alive" ( pinyin ''shēng'', Cantonese: ''saang''1), compared to four (, pinyin: ''sì'', Cantonese: ''sei''1), which sounds like the word "death" ( pinyin ''sǐ'', Cantonese: ''sei''2). The phrase " Third time's the charm" refers to the superstition that after two failures in any endeavor, a third attempt is more likely to succeed. However, some superstitions say the opposite, stating that luck, especially bad luck, is often said to "come in threes". One such superstition, called " Three on a Match", says that it is unlucky to be the third person to light a cigarette from the same match or lighter. This superstition is sometimes asserted to have originated among soldiers in the trenches of the First World War when a sniper might see the first light, take aim on the second and fire on the third.See also
* Cube (algebra) – (3 superscript) * Thrice * Third * Triad * Trio * Rule of three * ɜ, also known as Reversed epsilonReferences
*Wells, D. '' The Penguin Dictionary of Curious and Interesting Numbers'' London: Penguin Group. (1987): 46–48External links