Non-standard positional numeral systems here designates numeral systems that may be denoted positional systems, but that deviate in one way or another from the following description of standard positional systems:
- In a standard positional numeral system, the base b is a positive integer, and b different numerals are used to represent all non-negative integers. Each numeral represents one of the values 0, 1, 2, etc., up to b-1, but the value also depends on the position of the digit in a number. The value of a digit string like in base b is given by the polynomial form
- For instance, in hexadecimal (b=16), using A=10, B=11 etc., the digit string 1F3A means
- Introducing a radix point "." and a minus sign "–", all rational numbers can be represented.
This article summarizes facts on some non-standard positional numeral systems. In most cases, the polynomial form in the description of standard systems still applies.
Certain historical numeral systems like the Babylonian (standard) sexagesimal notation or the Chinese rod numerals could be classified as standard systems, if the 60 resp. 10 distinct numerals are considered as digits, unconventionally counting the space representing zero as a numeral. However, they could also be classified as non-standard systems (more specifically, mixed-base systems with unary components), if the primitive repeated glyphs making up the numerals are considered.
Bijective numeration systems
A bijective numeral system
with base b
different numerals to represent all non-negative integers. However, the numerals have values 1, 2, 3, etc. up to and including b
, where as zero is represented by an empty digit string. For example it is possible to have decimal without a zero
Base one (unary numeral system)
Unary is the bijective numeral system with base b
=1. In unary, one numeral is used to represent all positive integers. The value of the digit string
given by the polynomial form can be simplified into
for all n
. The non-standard features of this system are:
- The value of a digit does not depend on its position. Thus, one can easily argue that unary is not a positional system at all.
- Introducing a radix point in this system will not enable representation of non-integer values.
- The single numeral represents the value 1, not the value 0=b-1.
- The value 0 cannot be represented (or is implicitly represented by an empty digit string).
In some systems, while the base is a positive integer, negative digits are allowed. Non-adjacent form
is a particular system where the base is b
=2. In the balanced ternary
system, the base is b
=3, and the numerals have the values −1, 0 and +1 (rather than 0, 1 and 2 as in the standard ternary system
, or 1, 2 and 3 as in the bijective ternary system).
Bases that are not positive integers
A few positional systems have been suggested in which the base b
is not a positive integer.
Negative-base systems include negabinary
; in base −b
the number of different numerals used is b
In purely imaginary base bi
² numbers from 0 to |bi|
²-1 are used as digits.
It can be generalized on other complex bases: Complex base systems
In these systems, the number of different numerals used clearly cannot be b
. Example: Golden ratio base
It is sometimes convenient to consider positional numeral systems where the weights associated with the positions do not form a geometric sequence
, etc., starting from the least significant position, as given in the polynomial form. In a mixed radix
system such as the factoradic
system, the weights form a sequence where each weight is an integral multiple of the previous one. However, other sequences can be used, but then, every number does not necessarily have a unique representation, though unique representations may be guaranteed by imposing suitable constraints on the digit sequence. For example, using the Fibonacci sequence
(1, 2, 3, 5, 8, ...) and the digits 0 and 1 leads to Fibonacci coding
; requiring no consecutive 1's ensures a unique representation of all non-negative integers.
For calendrical use, the Mayan numeral system was a mixed radix system, since one of its positions represents a multiplication by 18 rather than 20, in order to fit a 360-day calendar. Also, giving an angle in degrees, minutes and seconds (with decimals), or a time in days, hours, minutes and seconds, can be interpreted as mixed radix systems.