Bijective numeration

The base-k bijective numeration system uses the digit-set {1, 2, ..., k} (k ≥ 1) to uniquely represent every non-negative integer, as follows: • The integer zero is represented by the empty string. • The integer represented by the nonempty digit-string :: :is ::. • The digit-string representing the integer m > 0 is :: :where :: \begin{align} a_0 & = m - q_0 k , & q_0 & = f\left(\frac m k \right) & \\ a_1 & = q_0 - q_1 k , & q_1 & = f\left(\frac {q_0} k \right) & \\ a_2 & = q_1 - q_2 k , & q_2 & = f\left(\frac {q_1} k \right) & \\ & \,\,\,\vdots & & \,\,\,\vdots \\ a_n & = q_{n-1} - 0 k , & q_n & = f\left(\frac {q_{n-1}} k \right) = 0 \end{align} :and ::f(x) = \lceil x \rceil - 1, :\lceil x \rceil being the least integer not less than x (the ceiling function). In contrast, standard positional notation can be defined with a similar recursive algorithm where ::f(x) = \lfloor x \rfloor, Extension to integers For base k > 1, the bijective base-k numeration system could be extended to negative integers in the same way as the standard base-b numeral system by use of an infinite number of the digit d_{k - 1}, where f(d_{k - 1}) = k - 1, represented as a left-infinite sequence of digits \ldots d_{k - 1}d_{k - 1}d_{k - 1} = \overline{d_{k - 1}}. This is because the Euler summation :g(\overline{d_{k - 1}}) = \sum_{i = 0}^\infty f(d_{k - 1}) k^i = -\frac{k - 1}{k - 1} = -1 meaning that :g(\overline{d_{k - 1}}d_{k}) = f(d_{k}) \sum_{i = 1}^\infty f(d_{k - 1}) k^i = 1 + \sum_{i = 0}^\infty f(d_{k - 1}) k^i = 0 and for every positive number n with bijective numeration digit representation d is represented by \overline{d_{k - 1}}d_{k}d. For base k > 2, negative numbers n are represented by \overline{d_{k - 1}}d_{i}d with i , while for base k = 2, negative numbers n are represented by \overline{d_{k}}d. This is similar to how in signed-digit representations, all integers n with digit representations d are represented as \overline{d_0}d where f(d_0) = 0. This representation is no longer bijective, as the entire set of left-infinite sequences of digits is used to represent the k-adic integers, of which the integers are only a subset. ==Properties of bijective base-k numerals==

For a given base k \geq 2, • the number of digits in the bijective base-k numeral representing a nonnegative integer n is • : \lfloor \log_k ((n+1)(k - 1))\rfloor, in contrast to \lceil \log_k(n+1)\rceil for ordinary base-k numerals;if k = 1 (i.e., unary), then the number of digits is just n; • the smallest nonnegative integer, representable in a bijective base-k numeral of length \ell \geq 0, is • : \mathrm{min}(\ell)=\frac{k^{\ell}-1}{k-1}; • the largest nonnegative integer, representable in a bijective base-k numeral of length \ell \geq 0, is • : \mathrm{max}(\ell)=\frac{k^{\ell+1}-k}{k-1}, equivalent to \mathrm{max}(\ell)=k \times \mathrm{min}(\ell), or \mathrm{max}(\ell)=\mathrm{min}(\ell+1)-1; • the bijective base-k and ordinary base-k numerals for a nonnegative integer n are identical if and only if the ordinary numeral does not contain the digit 0 (or, equivalently, the bijective numeral is neither the empty string nor contains the digit k). For a given base k \geq 1, • there are exactly k^{\ell} bijective base-k numerals of length \ell \geq 0; • a list of bijective base-k numerals, in natural order of the integers represented, is automatically in shortlex order (shortest first, lexicographical within each length). Thus, using λ to denote the empty string, the base 1, 2, 3, 8, 10, 12, and 16 numerals are as follows (where the ordinary representations are listed for comparison): ==Examples==

:34152 (in bijective base-5) = 3×54 + 4×53 + 1×52 + 5×51 + 2×1 = 2427 (in decimal). :119A (in bijective base-10, with "A" representing the digit value ten) = 1×103 + 1×102 + 9×101 + 10×1 = 1200 (in decimal). :A typical alphabetic list with more than 26 elements is bijective, using the order of A, B, C...X, Y, Z, AA, AB, AC...ZX, ZY, ZZ, AAA, AAB, AAC... ==The bijective base-10 system==

The bijective base-10 system is a base-ten positional numeral system that does not use a digit to represent zero. It instead has a digit to represent ten, such as A. As with conventional decimal, each digit position represents a power of ten, so for example 123 is "one hundred, plus two tens, plus three units". All positive integers that are represented solely with non-zero digits in conventional decimal (such as 123) have the same representation in the bijective base-10 system. Those that use a zero must be rewritten, so for example 10 becomes A, conventional 20 becomes 1A ("one ten, plus ten units"), conventional 100 becomes 9A, conventional 101 becomes A1, conventional 302 becomes 2A2, conventional 1000 becomes 99A, conventional 1110 becomes AAA, conventional 2010 becomes 19AA, and so on. Addition and multiplication in this system are essentially the same as with conventional decimal, except that carries occur when a position exceeds ten, rather than when it exceeds nine. So to calculate 643 + 759, there are twelve units (write 2 at the right and carry 1 to the tens), ten tens (write A with no need to carry to the hundreds), thirteen hundreds (write 3 and carry 1 to the thousands), and one thousand (write 1), to give the result 13A2 rather than the conventional 1402. ==The bijective base-26 system==

In the bijective base-26 system one may use the Latin alphabet letters "A" to "Z" to represent the 26 digit values one to twenty-six. (A=1, B=2, C=3, ..., Z=26) With this choice of notation, the number sequence (starting from 1) begins A, B, C, ..., X, Y, Z, AA, AB, AC, ..., AX, AY, AZ, BA, BB, BC, ... Each digit position represents a power of twenty-six, so for example, the numeral WI represents the value = 607 in base 10. Many spreadsheets including Microsoft Excel use this system to assign labels to the columns of a spreadsheet, starting A, B, C, ..., Z, AA, AB, ..., AZ, BA, ..., ZZ, AAA, etc. For instance, in Excel 2013, there can be up to 16384 columns (214 in binary code), labeled from A to XFD. Malware variants are also named using this system: for example, the first widespread Microsoft Word macro virus, Concept, is formally named WM/Concept.A, its 26th variant WM/Concept.Z, the 27th variant WM/Concept.AA, et seq. A variant of this system is used to name variable stars. It can be applied to any problem where a systematic naming using letters is desired, while using the shortest possible strings. ==Historical notes==

The fact that every non-negative integer has a unique representation in bijective base-k (k ≥ 1) is a "folk theorem" that has been rediscovered many times. Early instances are for the case k = 10, and and for all k ≥ 1. Smullyan uses this system to provide a Gödel numbering of the strings of symbols in a logical system; Böhm uses these representations to perform computations in the programming language P′′. mentions the special case of k = 10, and discusses the cases k ≥ 2. appears to be another rediscovery, and hypothesizes that if ancient numeration systems used bijective base-k, they might not be recognized as such in archaeological documents, due to general unfamiliarity with this system. ==Notes==