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A NUMERAL SYSTEM (or SYSTEM OF NUMERATION) is a writing system for expressing numbers; that is, a mathematical notation for representing numbers of a given set, using digits or other symbols in a consistent manner. It can be seen as the context that allows the symbols "11" to be interpreted as the binary symbol for _three_, the decimal symbol for _eleven_, or a symbol for other numbers in different bases .

The number the numeral represents is called its value.

Ideally, a numeral system will:

* Represent a useful set of numbers (e.g. all integers , or rational numbers ) * Give every number represented a unique representation (or at least a standard representation) * Reflect the algebraic and arithmetic structure of the numbers.

For example, the usual decimal representation of whole numbers gives every nonzero whole number a unique representation as a finite sequence of digits , beginning by a non-zero digit. However, when decimal representation is used for the rational or real numbers, such numbers in general have an infinite number of representations, for example 2.31 can also be written as 2.310, 2.3100000, 2.309999999..., etc., all of which have the same meaning except for some scientific and other contexts where greater precision is implied by a larger number of figures shown.

Numeral systems are sometimes called _number systems _, but that name is ambiguous, as it could refer to different systems of numbers, such as the system of real numbers , the system of complex numbers , the system of _p_-adic numbers , etc. Such systems are, however, not the topic of this article.

CONTENTS

* 1 Main numeral systems * 2 Positional systems in detail * 3 Generalized variable-length integers * 4 See also * 5 References * 6 Sources * 7 External links

MAIN NUMERAL SYSTEMS

Main article: List of numeral systems

The most commonly used system of numerals is the Hindu–Arabic numeral system . Two Indian mathematicians are credited with developing it. Aryabhata of Kusumapura developed the place-value notation in the 5th century and a century later Brahmagupta introduced the symbol for zero . The numeral system and the zero concept, developed by the Hindus in India, slowly spread to other surrounding countries due to their commercial and military activities with India. The Arabs adopted and modified it. Even today, the Arabs call the numerals which they use "Rakam Al-Hind" or the Hindu numeral system. The Arabs translated Hindu texts on numerology and spread them to the western world due to their trade links with them. The Western world modified them and called them the Arabic numerals, as they learned them from the Arabs. Hence the current western numeral system is the modified version of the Hindu numeral system developed in India. It also exhibits a great similarity to the Sanskrit–Devanagari notation, which is still used in India and neighbouring Nepal.

The simplest numeral system is the unary numeral system , in which every natural number is represented by a corresponding number of symbols. If the symbol / is chosen, for example, then the number seven would be represented by ///////. Tally marks represent one such system still in common use. The unary system is only useful for small numbers, although it plays an important role in theoretical computer science . Elias gamma coding , which is commonly used in data compression , expresses arbitrary-sized numbers by using unary to indicate the length of a binary numeral.

The unary notation can be abbreviated by introducing different symbols for certain new values. Very commonly, these values are powers of 10; so for instance, if / stands for one, − for ten and + for 100, then the number 304 can be compactly represented as +++ //// and the number 123 as + − − /// without any need for zero. This is called sign-value notation . The ancient Egyptian numeral system was of this type, and the Roman numeral system was a modification of this idea.

More useful still are systems which employ special abbreviations for repetitions of symbols; for example, using the first nine letters of the alphabet for these abbreviations, with A standing for "one occurrence", B "two occurrences", and so on, one could then write C+ D/ for the number 304. This system is used when writing Chinese numerals and other East Asian numerals based on Chinese. The number system of the English language is of this type ("three hundred four"), as are those of other spoken languages , regardless of what written systems they have adopted. However, many languages use mixtures of bases, and other features, for instance 79 in French is _soixante dix-neuf_ (60 + 10 + 9) and in Welsh is _pedwar ar bymtheg a thrigain_ (4 + (5 + 10) + (3 × 20)) or (somewhat archaic) _pedwar ugain namyn un_ (4 × 20 − 1). In English, one could say "four score less one", as in the famous Gettysburg Address representing "87 years ago" as "four score and seven years ago".

More elegant is a _positional system _, also known as place-value notation. Again working in base 10, ten different digits 0, ..., 9 are used and the position of a digit is used to signify the power of ten that the digit is to be multiplied with, as in 304 = 3×100 + 0×10 + 4×1 or more precisely 3×102 + 0×101 + 4×100. Note that zero, which is not needed in the other systems, is of crucial importance here, in order to be able to "skip" a power. The Hindu–Arabic numeral system, which originated in India and is now used throughout the world, is a positional base-10 system.

Arithmetic is much easier in positional systems than in the earlier additive ones; furthermore, additive systems need a large number of different symbols for the different powers of 10; a positional system needs only ten different symbols (assuming that it uses base 10).

Positional decimal system is presently universally used in human writing. The base 1000 is also used, by grouping the digits and considering a sequence of three decimal digits as a single digit. This is the meaning of the common notation 1,000,234,567 used for very large numbers.

In computers , the main numeral systems are based on the positional system in base 2 (binary numeral system ), with two binary digits , 0 and 1. Positional systems obtained by grouping binary digits by three (octal numeral system ) or four (hexadecimal numeral system ) are commonly used. For very large integers, bases 232 or 264 (grouping binary digits by 32 or 64, the length of the machine word ) are used, as, for example, in GMP .

The numerals used when writing numbers with digits or symbols can be divided into two types that might be called the arithmetic numerals (0,1,2,3,4,5,6,7,8,9) and the geometric numerals (1, 10, 100, 1000, 10000 ...), respectively. The sign-value systems use only the geometric numerals and the positional systems use only the arithmetic numerals. A sign-value system does not need arithmetic numerals because they are made by repetition (except for the Ionic system ), and a positional system does not need geometric numerals because they are made by position. However, the spoken language uses _both_ arithmetic and geometric numerals.

In certain areas of computer science, a modified base-_k_ positional system is used, called bijective numeration , with digits 1, 2, ..., _k_ (_k_ ≥ 1), and zero being represented by an empty string. This establishes a bijection between the set of all such digit-strings and the set of non-negative integers, avoiding the non-uniqueness caused by leading zeros. Bijective base-_k_ numeration is also called _k_-adic notation, not to be confused with _p_-adic numbers . Bijective base-1 is the same as unary.

POSITIONAL SYSTEMS IN DETAIL

See also: Positional notation

In a positional base-_b_ numeral system (with _b_ a natural number greater than 1 known as the radix ), _b_ basic symbols (or digits) corresponding to the first _b_ natural numbers including zero are used. To generate the rest of the numerals, the position of the symbol in the figure is used. The symbol in the last position has its own value, and as it moves to the left its value is multiplied by _b_.

For example, in the decimal system (base 10), the numeral 4327 means (4×103) + (3×102) + (2×101) + (7×100), noting that 100 = 1.

In general, if _b_ is the base, one writes a number in the numeral system of base _b_ by expressing it in the form _a__n__b__n_ + _a__n_ − 1_b__n_ − 1 + _a__n_ − 2_b__n_ − 2 + ... + _a_0_b_0 and writing the enumerated digits _a_n_a__n_ − 1_a__n_ − 2 ... _a_0 in descending order. The digits are natural numbers between 0 and _b_ − 1, inclusive.

If a text (such as this one) discusses multiple bases, and if ambiguity exists, the base (itself represented in base 10) is added in subscript to the right of the number, like this: numberbase. Unless specified by context, numbers without subscript are considered to be decimal.

By using a dot to divide the digits into two groups, one can also write fractions in the positional system. For example, the base-2 numeral 10.11 denotes 1×21 + 0×20 + 1×2−1 + 1×2−2 = 2.75.

In general, numbers in the base _b_ system are of the form: ( a n a n 1 a 1 a 0 . c 1 c 2 c 3 ) b = k = 0 n a k b k + k = 1 c k b k . {displaystyle (a_{n}a_{n-1}cdots a_{1}a_{0}.c_{1}c_{2}c_{3}cdots )_{b}=sum _{k=0}^{n}a_{k}b^{k}+sum _{k=1}^{infty }c_{k}b^{-k}.}

The numbers _b__k_ and _b_−_k_ are the weights of the corresponding digits. The position _k_ is the logarithm of the corresponding weight _w_, that is k = log b w = log b b k {displaystyle k=log _{b}w=log _{b}b^{k}} . The highest used position is close to the order of magnitude of the number.

The number of tally marks required in the unary numeral system for _describing the weight_ would have been W. In the positional system, the number of digits required to describe it is only k + 1 = {displaystyle k+1=} _ LOG B W {DISPLAYSTYLE LOG _{B}W} + 1 {displaystyle +1} , for k_ ≥ 0. For example, to describe the weight 1000 then four digits are needed because log 10 1000 + 1 = 3 + 1 {displaystyle log _{10}1000+1=3+1} _. The number of digits required to describe the position_ is log b k + 1 = log b log b w + 1 {displaystyle log _{b}k+1=log _{b}log _{b}w+1} (in positions 1, 10, 100,... only for simplicity in the decimal example).

POSITION 3 2 1 0 −1 −2 . . .

WEIGHT b 3 {displaystyle b^{3}} b 2 {displaystyle b^{2}} b 1 {displaystyle b^{1}} b 0 {displaystyle b^{0}} b 1 {displaystyle b^{-1}} b 2 {displaystyle b^{-2}} {displaystyle dots }

DIGIT a 3 {displaystyle a_{3}} a 2 {displaystyle a_{2}} a 1 {displaystyle a_{1}} a 0 {displaystyle a_{0}} c 1 {displaystyle c_{1}} c 2 {displaystyle c_{2}} {displaystyle dots }

DECIMAL EXAMPLE WEIGHT 1000 100 10 1 0.1 0.01 . . .

DECIMAL EXAMPLE DIGIT 4 3 2 7 0 0 . . .

Note that a number has a terminating or repeating expansion if and only if it is rational ; this does not depend on the base. A number that terminates in one base may repeat in another (thus 0.310 = 0.0100110011001...2). An irrational number stays aperiodic (with an infinite number of non-repeating digits) in all integral bases. Thus, for example in base 2, π = 3.1415926...10 can be written as the aperiodic 11.001001000011111...2.

Putting overscores , _n_, or dots, _ṅ_, above the common digits is a convention used to represent repeating rational expansions. Thus: 14/11 = 1.272727272727... = 1.27 or 321.3217878787878... = 321.32178.

If _b_ = _p_ is a prime number , one can define base-_p_ numerals whose expansion to the left never stops; these are called the _p_-adic numbers .

GENERALIZED VARIABLE-LENGTH INTEGERS

More general is using a mixed radix notation (here writte