Signed-digit representation

In mathematical notation for numbers, a signed-digit representation is a positional numeral system with a set of signed digits used to encode the integers.

Signed-digit representation can be used to accomplish fast addition of integers because it can eliminate chains of dependent carries.[1] In the binary numeral system, a special case signed-digit representation is the non-adjacent form, which can offer speed benefits with minimal space overhead.

HistoryEdit

Challenges in calculation stimulated early authors Colson (1726) and Cauchy (1840) to use signed-digit representation. The further step of replacing negated digits with new ones was suggested by Selling (1887) and Cajori (1928).

In 1928, Florian Cajori noted the recurring theme of signed digits, starting with Colson (1726) and Cauchy (1840).[2] In his book History of Mathematical Notations, Cajori titled the section "Negative numerals".[3] For completeness, Colson[4] uses examples and describes addition (pp 163,4), multiplication (pp 165,6) and division (pp 170,1) using a table of multiples of the divisor. He explains the convenience of approximation by truncation in multiplication. Colson also devised an instrument (Counting Table) that calculated using signed digits.

Eduard Selling[5] advocated inverting the digits 1, 2, 3, 4, and 5 to indicate the negative sign. He also suggested snie, jes, jerd, reff, and niff as names to use vocally. Most of the other early sources used a bar over a digit to indicate a negative sign for a it. Another German usage of signed-digits was described in 1902 in Klein's encyclopedia.[6]

Definition and propertiesEdit

Digit setEdit

Let   be a finite set of numerical digits with cardinality   (If  , then the positional number system is trivial and only represents the trivial ring), with each digit denoted as   for  .   is known as the radix or number base.   can be used for a signed-digit representation if there exists a unique function   such that  .

  can be partitioned into three distinct sets  ,  , and  , representing the positive, zero, and negative digits respectively, such that all digits   satisfy  , all digits   satisfy   and all digits   satisfy  . The cardinality of   is  , the cardinality of   is  , and the cardinality of   is  , giving the number of positive and negative digits respectively, such that  .

Balanced form representationsEdit

Balanced form representations are representations where for every positive digit  , there exist a corresponding negative digit   such that  . It follows that  . Only odd bases can have balanced form representations, as when  ,   is always an odd number. In balanced form, the negative digits   are usually denoted as positive digits with a bar over the digit, as   for  . For example, the digit set of balanced ternary would be   with  ,  , and  . This convention is adopted in finite fields of odd prime order  :[7]

 

Dual signed-digit representationEdit

Every digit set   has a dual digit set   given by the inverse order of the digits with an isomorphism   defined by  . As a result, for any signed-digit representations   of a number system ring   constructed from   with valuation  , there exists a dual signed-digit representations of  ,  , constructed from   with valuation  , and an isomorphism   defined by  , where   is the additive inverse operator of  . The digit set for balanced form representations is self-dual.

For integersEdit

Given the digit set   and function   as defined above, let us define an integer endofunction   as the following:

 

If the only periodic point of   is the fixed point  , then the set of all signed-digit representations of the integers   using   is given by the Kleene plus  , the set of all finite concatenated strings of digits   with at least one digit, with  . Each signed-digit representation   has a valuation  

 .

Examples include balanced ternary with digits  .

Otherwise, if there exist a non-zero periodic point of  , then there exist integers that are represented by an infinite number of non-zero digits in  . Examples include the standard decimal numeral system with the digit set  , which requires an infinite number of the digit   to represent the additive inverse  , as  , and the positional numeral system with the digit set   with  , which requires an infinite number of the digit   to represent the number  , as  .

For decimal fractionsEdit

If the integers can be represented by the Kleene plus  , then the set of all signed-digit representations of the decimal fractions, or  -adic rationals  , is given by  , the Cartesian product of the Kleene plus  , the set of all finite concatenated strings of digits   with at least one digit, the singleton   consisting of the radix point (  or  ), and the Kleene star  , the set of all finite concatenated strings of digits  , with  . Each signed-digit representation   has a valuation  

 

For real numbersEdit

If the integers can be represented by the Kleene plus  , then the set of all signed-digit representations of the real numbers   is given by  , the Cartesian product of the Kleene plus  , the set of all finite concatenated strings of digits   with at least one digit, the singleton   consisting of the radix point (  or  ), and the Cantor space  , the set of all infinite concatenated strings of digits  , with  . Each signed-digit representation   has a valuation  

 .

The infinite series always converges to a finite real number.

For other number systemsEdit

All base-  numerals can be represented as a subset of  , the set of all doubly infinite sequences of digits in  , where   is the set of integers, and the ring of base-  numerals is represented by the formal power series ring  , the doubly infinite series

 

where   for  .

Integers modulo  Edit

The set of all signed-digit representations of the integers modulo  ,   is given by the set  , the set of all finite concatenated strings of digits   of length  , with  . Each signed-digit representation   has a valuation  

 

Prüfer groupsEdit

A Prüfer group is the quotient group   of the integers and the  -adic rationals. The set of all signed-digit representations of the Prüfer group is given by the Kleene star  , the set of all finite concatenated strings of digits  , with  . Each signed-digit representation   has a valuation  

 

Circle groupEdit

The circle group is the quotient group   of the integers and the real numbers. The set of all signed-digit representations of the circle group is given by the Cantor space  , the set of all right-infinite concatenated strings of digits  . Each signed-digit representation   has a valuation  

 

The infinite series always converges.

 -adic integersEdit

The set of all signed-digit representations of the  -adic integers,   is given by the Cantor space  , the set of all left-infinite concatenated strings of digits  . Each signed-digit representation   has a valuation  

 

 -adic solenoidsEdit

The set of all signed-digit representations of the  -adic solenoids,   is given by the Cantor space  , the set of all doubly infinite concatenated strings of digits  . Each signed-digit representation   has a valuation  

 

In written and spoken languageEdit

The oral and written forms of numbers in the Punjabi language use a form of a negative numeral one written as una or un.[8] This negative one is used to form 19, 29, …, 89 from the root for 20, 30, …, 90. Explicitly, here are the numbers:

  • 19 unni, 20 vih, 21 ikki
  • 29 unatti, 30 tih, 31 ikatti
  • 39 untali, 40 chali, 41 iktali
  • 49 unanja, 50 panjah, 51 ikvanja
  • 59 unahat, 60 sath, 61 ikahat
  • 69 unattar, 70 sattar, 71 ikhattar
  • 79 unasi, 80 assi, 81 ikiasi
  • 89 unanve, 90 nabbe, 91 ikinnaven.

Similarly, the Sesotho language utilizes negative numerals to form 8's and 9's.

  • 8 robeli (/Ro-bay-dee/) meaning "break two" i.e. two fingers down
  • 9 robong (/Ro-bong/) meaning "break one" i.e. one finger down

In the English language it is common to refer to times as, for example, 'seven til three', 'til' performing the negation.

Other systemsEdit

There exist other signed-digit bases such that the base  . A notable examples of this is Booth encoding, which has a digit set   with   and  , but which uses a base  . The standard binary numeral system would only use digits of value  .

Note that non-standard signed-digit representations are not unique. For instance:

 
 
 
 

The non-adjacent form (NAF) of Booth encoding does guarantee a unique representation for every integer value. However, this only applies for integer values. For example, consider the following repeating binary numbers in NAF,

 

See alsoEdit

Notes and referencesEdit

  1. ^ Dhananjay Phatak, I. Koren (1994) Hybrid Signed-Digit Number Systems: A Unified Framework for Redundant Number Representations with Bounded Carry Propagation Chains
  2. ^ Augustin-Louis Cauchy (16 Nov 1840) "Sur les moyens d'eviter les erreurs dans les calculs numerique", Comptes rendus 11:789. Also found in Oevres completes Ser. 1, vol. 5, pp. 434–42.
  3. ^ Cajori, Florian (1993) [1928-1929]. A History of Mathematical Notations. Dover Publications. p. 57. ISBN 978-0486677668.
  4. ^ John Colson (1726) "A Short Account of Negativo-Affirmativo Arithmetik", Philosophical Transactions of the Royal Society 34:161–173. Available as Early Journal Content from JSTOR
  5. ^ Eduard Selling (1887) Eine neue Rechenmachine, pp. 15–18, Berlin
  6. ^ Rudolf Mehmke (1902) "Numerisches Rechen", §4 Beschränkung in den verwendeten Ziffern, Klein's encyclopedia, I-2, p. 944.
  7. ^ Hirschfeld, J. W. P. (1979). Projective Geometries Over Finite Fields. Oxford University Press. p. 8. ISBN 978-0-19-850295-1.
  8. ^ Punjabi numbers from Quizlet