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.
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 non zero whole number a unique representation as a finite sequence of digits, beginning by a nonzero 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 padic numbers, etc. Such systems are not the topic of this article.
Contents

Main numeral systems 1

Positional systems in detail 2

Generalized variablelength integers 3

See also 4

References 5

Sources 6

External links 7
Main numeral systems
The most commonly used system of numerals is the Hindu–Arabic numeral system, based on Arabic numerals. Two Indian mathematicians are credited with developing it. Aryabhata of Kusumapura developed the placevalue notation in the 5th century and a century later Brahmagupta introduced the symbol for zero.^{[1]} 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 they use "Rakam AlHind" 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 from them. 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.
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 arbitrarysized 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 signvalue 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 [and] 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 dixneuf (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 placevalue notation. Again working in base10, 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 3×10^{2} + 0×10^{1} + 4×10^{0}. 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 base10 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 2^{32} or 2^{64} (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 signvalue systems use only the geometric numerals and the positional systems use only the arithmetic numerals. A signvalue 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 basek 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 digitstrings and the set of nonnegative integers, avoiding the nonuniqueness caused by leading zeros. Bijective basek numeration is also called kadic notation, not to be confused with padic numbers. Bijective base1 is the same as unary.
Positional systems in detail
In a positional baseb 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 (base10), the numeral 4327 means (4×10^{3}) + (3×10^{2}) + (2×10^{1}) + (7×10^{0}), noting that 10^{0} = 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 base10) is added in subscript to the right of the number, like this: number_{base}. 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 base2 numeral 10.11 denotes 1×2^{1} + 0×2^{0} + 1×2^{−1} + 1×2^{−2} = 2.75.
In general, numbers in the base b system are of the form:

(a_na_{n1}\cdots a_1a_0.c_1 c_2 c_3\cdots)_b = \sum_{k=0}^n a_kb^k + \sum_{k=1}^\infty c_kb^{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. 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 = \log_{b} w + 1, for k ≥ 0. For example, to describe the weight 1000 then four digits are needed because \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 (in positions 1, 10, 100,... only for simplicity in the decimal example).
Position

3

2

1

0

−1

−2

. . .

Weight

b^3

b^2

b^1

b^0

b^{1}

b^{2}

\dots

Digit

a_3

a_2

a_1

a_0

c_1

c_2

\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 0.3_{10} = 0.0100110011001..._{2}). An irrational number stays aperiodic (with an infinite number of nonrepeating digits) in all integral bases. Thus, for example in base2, 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 basep numerals whose expansion to the left never stops; these are called the padic numbers.
Generalized variablelength integers
More general is using a mixed radix notation (here written littleendian) like a_0 a_1 a_2 for a_0 + a_1 b_1 + a_2 b_1 b_2, etc.
This is used in punycode, one aspect of which is the representation of a sequence of nonnegative integers of arbitrary size in the form of a sequence without delimiters, of "digits" from a collection of 36: a–z and 0–9, representing 0–25 and 26–35 respectively. A digit lower than a threshold value marks that it is the mostsignificant digit, hence the end of the number. The threshold value depends on the position in the number. For example, if the threshold value for the first digit is b (i.e. 1) then a (i.e. 0) marks the end of the number (it has just one digit), so in numbers of more than one digit the range is only b–9 (1–35), therefore the weight b_{1} is 35 instead of 36. Suppose the threshold values for the second and third digits are c (2), then the third digit has a weight 34 × 35 = 1190 and we have the following sequence:
a (0), ba (1), ca (2), .., 9a (35), bb (36), cb (37), .., 9b (70), bca (71), .., 99a (1260), bcb (1261), etc.
Unlike a regular based numeral system, there are numbers like 9b where 9 and b each represents 35; yet the representation is unique because ac and aca are not allowed – the a would terminate the number.
The flexibility in choosing threshold values allows optimization depending on the frequency of occurrence of numbers of various sizes.
The case with all threshold values equal to 1 corresponds to bijective numeration, where the zeros correspond to separators of numbers with digits which are nonzero.
See also
References

^ David Eugene Smith; Louis Charles Karpinski (1911). The HinduArabic numerals. Ginn and Company.
Sources

Georges Ifrah. The Universal History of Numbers : From Prehistory to the Invention of the Computer, Wiley, 1999. ISBN 0471375683.

D. Knuth. The Art of Computer Programming. Volume 2, 3rd Ed. Addison–Wesley. pp. 194–213, "Positional Number Systems".

A.L. Kroeber (Alfred Louis Kroeber) (1876–1960), Handbook of the Indians of California, Bulletin 78 of the Bureau of American Ethnology of the Smithsonian Institution (1919)

J.P. Mallory and D.Q. Adams, Encyclopedia of IndoEuropean Culture, Fitzroy Dearborn Publishers, London and Chicago, 1997.

Hans J. Nissen; Peter Damerow; Robert K. Englund (1993). Archaic Bookkeeping: Early Writing and Techniques of Economic Administration in the Ancient Near East. University Of Chicago Press.

SchmandtBesserat, Denis (1996). How Writing Came About. University of Texas Press.

Zaslavsky, Claudia (1999). Africa counts: number and pattern in African cultures. Chicago Review Press.
External links

Numerical Mechanisms and Children's Concept of Numbers

Software for converting from one numeral system to another

Online conversion of fractional numbers between numeral systems

Open source numeral systems converter

Open source numeral systems calculator

Online multi numeral system converter
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