Decimal, also called denary, is the base10numeral system, which uses the symbols 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 (called digits) together with the decimal point and the sign symbols + (plus) and − (minus) to represent numbers.
Decimal is the principal numeral system used by humans (though some cultures do or did use other number systems). This is almost certainly because humans have ten fingers; digit is also the anatomical term referring to fingers and toes. The set of symbols for the digits is called Arabic numerals by Europeans and Hindi numerals by Arabs, each term referring to the people that the users of the term got it from.
Computers often use other numeral systems (notably binary, octal, and hexadecimal), because it is slightly more efficient to implement these power-of-two systems electronically. Decimal numerals can be encoded for computers using binary-coded decimal or more efficient schemes.
Decimal fractions are usually expressed without a denominator, the decimal point being inserted into the numerator at a position corresponding to the power of ten of the denominator. E.g. 8/10, 833/100, 83/1000, 8/10000 and 80/10000 are expressed thus: 0.8, 8.33, 0.083, 0.0008 & 0.008.
The integer and fractional parts of a decimal number are separated by a decimal point. In this article, as in most of the English speaking world, a dot (.) is used. It is usual for a decimal number which is less than one to have a leading zero. Trailling zeroes after the decimal point are not necessary, although in science, engineering and statistics they can be retained to show a level of confidence in the accuracy of the number: Whereas 0.080 and 0.08 are mathematically the same number, in engineering 0.080 suggests an error of up to 1 part in a thousand, while 0.08 suggests an error of up to 1 in a 100.
Ten is the product of the first and third prime number, is one greater than the square of the second prime number, and is one less than the fifth prime number. This leads to plenty of simple decimal fractions:
1/2 = 0.5
1/3 = 0.333333... (with 3 recurring)
1/4 = 0.25
1/5 = 0.2
1/6 = 0.166666... (with 6 recurring)
1/8 = 0.125
1/9 = 0.111111... (with 1 recurring)
1/10 = 0.1
1/11 = 0.090909... (with 09 recurring)
1/12 = 0.083333... (with 3 recurring)
Other prime factors in the denominator will give longer recurring sequences, see for instance 7, 13.
That a rational must producing a finite or recurring decimal expansion can be seen to be a consequence of the long division algorithm, in that there are only (q-1) possible nonzero remainders on division by q, so that the recurring pattern will have a period less than q-1. For instance to find 3/7 by long division:
The converse to this observation is that every recurring decimal represents a rational number p/q. This is a consequence of the fact the recurring part of a decimal representation is, in fact, an infinite geometric series which will sum to a rational number. For instance,
The representation is unique, except for rational numbers which can be written as p/(2a5b) (i.e. the only prime factors in denominator are 2 and 5). In all such cases there is a terminating decimal representation. For instance 1/1=1, −1/2=−0.5, 3/5=0.6, 3/25=0.12 and 1306/1250=1.0448. Such numbers are the only real numbers which don't have a unique decimal representation, as they can also be written as a representation that has a recurring 9. For instance 1=0.99999..., −1/2=−0.499999..., etc.
Rational numbers p/q with prime factors in the denominator other than 2 and 5 (when reduced to simplest terms) have a unique recurring decimal representation.
This leaves the irrational numbers. They also have unique infinite decimal representation, and can be characterised as the numbers whose decimal representations neither terminate nor recur.