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. For example, the number 21 is divisible by three (3 times 7) and the sum of its digits is 2 + 1 = 3. If n is greater than 2, both n − 1 and n + 1 are greater than 1 so their product is not prime.Ī natural number is divisible by three if the sum of its digits in base 10 is divisible by 3. This is true for 3 as well (with n = 2), but in this case the smaller factor is 1. Any other number which is n 2 − 1 for some integer n is not prime, since it is ( n − 1)( n + 1). Three is the only prime which is one less than a perfect square.

the only known number n such that n!-1 and n!+1 are prime.

the smallest number of sides that a simple (non-self-intersecting) polygon can have.

the fourth unique Fibonacci number and the fifth in the sequence.

the first odd prime number and the second smallest prime.

the number of non-collinear points needed to determine a plane and a circle.a rough approximation of π (3.1415.) and a very rough approximation of e (2.71828.) when doing quick estimates.It is found on UPC-A barcodes and standard 52-card decks. This form is sometimes used to prevent falsifying a 3 as an 8.

In some French text-figure typefaces, though, it has an ascender instead of a descender.Ī common graphic variant of the digit three has a flat top, similar to the letter Ʒ (ezh). In typefaces with text figures, on the other hand, the glyph usually has the height of a lowercase letter 'x' and a descender: ' '. In most modern Western typefaces, the digit 3, like the other decimal digits, has the height of a capital letter, and sits on the baseline. Text figures in various fonts: Adobe Garamond, Adobe Caslon, Theano Didot and Essonnes Text In contrast, the Eastern Arabs retained and enlarged that stroke, rotating the digit once more to yield the modern ('Eastern') Arabic digit ' ٣'. The bottom stroke was dropped around the 10th century in the western parts of the Caliphate, such as the Maghreb and Al-Andalus, when a distinct variant ('Western Arabic') of the digit symbols developed, including modern Western 3. The Indian digits spread to the Caliphate in the 9th century. In cursive script the three strokes were eventually connected to form a glyph resembling a ⟨3⟩ with an additional stroke at the bottom: ३.

The Nāgarī script rotated the lines clockwise, so they appeared horizontally, and ended each line with a short downward stroke on the right. However, during the Gupta Empire the sign was modified by the addition of a curve on each line. That was also the original representation of 3 in the Brahmic (Indian) numerical notation, its earliest forms aligned vertically. The use of three lines to denote the number 3 occurred in many writing systems, including some (like Roman and Chinese numerals) that are still in use.

Numerology and the divine triangle used