In mathematics and in particular in combinatorics, the Lehmer code is a particular way to encode each possible permutation of a sequence of n numbers. It is an instance of a scheme for numbering permutations and is an example of an inversion table. The Lehmer code is named in reference to D. H. Lehmer, but the code had been known since 1888 at least.
The code The Lehmer code makes use of the fact that there are
n ! = n × ( n − 1 ) × ⋯ × 2 × 1 {\displaystyle n!=n\times (n-1)\times \cdots \times 2\times 1}
permutations of a sequence of n numbers. If a permutation σ is specified by the sequence (σ1, ..., σn) of its images of 1, ..., n, then it is encoded by a sequence of n numbers, but not all such sequences are valid since every number must be used only once. By contrast the encodings considered here choose the first number from a set of n values, the next number from a fixed set of n − 1 values, and so forth decreasing the number of possibilities until the last number for which only a single fixed value is allowed; every sequence of numbers chosen from these sets encodes a single permutation. While several encodings can be defined, the Lehmer code has several additional useful properties; it is the sequence
L ( σ ) = ( L ( σ ) 1 , … , L ( σ ) n ) where L ( σ ) i = # { j > i : σ j < σ i } , {\displaystyle L(\sigma )=(L(\sigma )_{1},\ldots ,L(\sigma )_{n})\quad {\text{where}}\quad L(\sigma )_{i}=\#\{j>i:\sigma _{j}<\sigma _{i}\},}
in other words the term L(σ)i counts the number of terms in (σ1, ..., σn) to the right of σi that are smaller than it, a number between 0 and n − i, allowing for n + 1 − i different values. A pair of indices (i,j) with i < j and σi > σj is called an inversion of σ, and L(σ)i counts the number of inversions (i,j) with i fixed and varying j. It follows that L(σ)1 + L(σ)2 + … + L(σ)n is the total number of inversions of σ, which is also the number of adjacent transpositions that are needed to transform the permutation into the identity permutation. Other properties of the Lehmer code include that the lexicographical order of the encodings of two permutations is the same as that of their sequences (σ1, ..., σn), that any value 0 in the code represents a right-to-left minimum in the permutation (i.e., a σi smaller than any σj to its right), and a value n − i at position i similarly signifies a right-to-left maximum, and that the Lehmer code of σ coincides with the factorial number system representation of its position in the list of permutations of n in lexicographical order (numbering the positions starting from 0). Variations of this encoding can be obtained by counting inversions (i,j) for fixed j rather than fixed i, by counting inversions with a fixed smaller value σj rather than smaller index i, or by counting non-inversions rather than inversions; while this does not produce a fundamentally different type of encoding, some properties of the encoding will change correspondingly. In particular counting inversions with a fixed smaller value σj gives the inversion table of σ, which can be seen to be the Lehmer code of the inverse permutation.
Encoding and decoding The usual way to prove that there are n! different permutations of n objects is to observe that the first object can be chosen in n different ways, the next object in n − 1 different ways (because choosing the same number as the first is forbidden), the next in n − 2 different ways (because there are now 2 forbidden values), and so forth. Translating this freedom of choice at each step into a number, one obtains an encoding algorithm, one that finds the Lehmer code of a given permutation. One need not suppose the objects permuted to be numbers, but one needs a total ordering of the set of objects. Since the code numbers are to start from 0, the appropriate number to encode each object σi by is the number of objects that were available at that point (so they do not occur before position i), but which are smaller than the object σi actually chosen. (Inevitably such objects must appear at some position j > i, and (i,j) will be an inversion, which shows that this number is indeed L(σ)i.) This number to encode each object can be found by direct counting, in several ways (directly counting inversions, or correcting the total number of objects smaller than a given one, which is its sequence number starting from 0 in the set, by those that are unavailable at its position). Another method which is in-place, but not really more efficient, is to start with the permutation of {0, 1, ... n − 1} obtained by representing each object by its mentioned sequence number, and then for each entry x, in order from left to right, correct the items to its right by subtracting 1 from all entries (still) greater than x (to reflect the fact that the object corresponding to x is no longer available). Concretely a Lehmer code for the permutation B,F,A,G,D,E,C of letters, ordered alphabetically, would first give the list of sequence numbers 1,5,0,6,3,4,2, which is successively transformed
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