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A number in a positional number system is represented as an expansion
where
is the radix (or base) with ,
is the exponent (position or place) ,
are digits from the finite set of digits , usually with
The cardinality is called the level of decomposition.
A positional number system or coding system is a pair
with radix and set of digits , and we write the standard set of digits with digits as
Desirable are coding systems with the features:
Every number in , e. g. the integers , the Gaussian integers or the integers , is uniquely representable as a finite code, possibly with a sign ±.
Every number in the field of fractions, which possibly is completed for the metric given by yielding or , is representable as an infinite series which converges under for , and the measure of the set of numbers with more than one representation is 0. The latter requires that the set be minimal, i. e. for real resp.[clarify] for complex numbers.
In the real numbers
In this notation our standard decimal coding scheme is denoted by
Binary coding systems of complex numbers, i. e. systems with the digits , are of practical interest.[9]
Listed below are some coding systems (all are special cases of the systems above) and codes for the numbers −1, 2, −2, .
The standard binary (which requires a sign) and the "negabinary" systems are also listed for comparison. They do not have a genuine expansion for .
If the set of digits is minimal, the set of such numbers has a measure of 0. This is the case with all the mentioned coding systems.
Base −1 ± i
Of particular interest, the quater-imaginary base (base 2i) and base −1±i systems discussed below can be used to finitely represent the Gaussian integers without sign.
Base −1±i, using digits 0 and 1, was proposed by S. Khmelnik in 1964[3] and Walter F. Penney in 1965.[4][6] The rounding region of an integer – i.e., a set of complex (non-integer) numbers that share the integer part of their representation in this system – has a fractal shape, the twindragon.
Example: 3 = 112; 11−1+i = i; the position of 3 on the graph (x, y·i) is (0, 1).
^Khmelnik, S.I. (1966). "Positional coding of complex numbers". Questions of Radio Electronics (in Russian). XII (9).
^ abKhmelnik, S.I. (2004 (see also here)). Coding of Complex Numbers and Vectors (in Russian). «Mathematics in Computers», Israel, ISBN 978-0-557-74692-7. {{cite book}}: Check date values in: |year= (help); External link in |year= (help)