#### In quantum computing, a **qubit** or **quantum bit** (sometimes **qbit**) is the basic unit of quantum information: 3

3 (three) is a number, numeral and digit. It is the natural number following 2 and preceding 4, and is the smallest odd prime number. It has religious or cultural significance in many societies.

3 is:

- a rough approximation of
*π*(3.1415…) and a very rough approximation of*e*(2.71828..) when doing quick estimates. - the number of non-collinear points needed to determine a plane and a circle.
- the first odd prime number and the second smallest prime.
- the first Fermat prime (2
^{2n}+ 1). - the first Mersenne prime (2
^{n}− 1). - the second Sophie Germain prime.
- the second Mersenne prime exponent.
- the second factorial prime (2! + 1).
- the second Lucas prime.
- the second triangular number. It is the only prime triangular number.
- the fourth Fibonacci number.
- the smallest number of sides that a simple (non-self-intersecting) polygon can have.

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. That was also the original representation of 3 in the Brahmic (Indian) numerical notation. However, during the Gupta Empire the sign was modified by the addition of a curve on each line. The Nagari rotated the lines clockwise[clarification needed], ended each line with a short downward stroke on the right. 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 Indian digits spread to the Caliphate in the 9th century. 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. In contrast, the Eastern Arabs retained and enlarged that stroke, rotating the digit once more to yield the modern (“Eastern”) Arabic digit “٣”.

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