Lexicographic and colexicographic order

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Lexicographic (Lex) and colexicographic (CoLex) order are probably the most important ways to order tuples in mathematics.

Lex order is that of a dictionary.
CoLex order is obtained by reflecting all tuples, applying Lex order, and reflecting the tuples again.

Lex order is more intuitive for most people.
CoLex order is more practical when the finite sets of tuples to be ordered shall be generalized to infinite sets of sequences.

Both orderings can be reversed, so there are actually four different orderings.
They can also be reflected (see here), but that's not a different ordering of the set of tuples, but just a certain ordering written in a different way.
When the set of tuples contains all reflections, each reflected order is equal to some of the four others (see below).


The sequence of the k-subsets of in CoLex order
is the beginning of the infinite sequence of k-subsets of in CoLex order.
This corresponds to the increasing sequence Sloane'sA014311 = 7,11,13,14,19,21...

In the file on the left it can be seen that the blue patterns are horizontally reflected.
However, this is not the case in the file on the right, which shows only some of the subsets.

The 3-subsets of
The 10 3-subsets of with an even sum of elements
The 3-subsets of
(binary vectors RevLex, combinations Lex)
This sequence of combinations is not the beginning of the sequence of combinations in Lex order in the file on the left.

2-subsets of the integers

The triangle on the right shows 2-subsets of the integers.
This corresponds to the sequence Sloane'sA018900 as a square array.
The sequence of numbers (3,5,6,9,10,12...) corresponds to the CoLex ordering of 2-subsets:

    1     2
    1     3
    2     3
    1     4
    2     4
    3     4
    1     5
    2     5
    3     5
    4     5
    1     6
    2     6
    3     6
    4     6
    5     6
    1     7
    2     7
    3     7
    4     7
    5     7
    6     7
    1     8
    2     8
    3     8
    4     8
    5     8
    6     8
    7     8


Finite sets of permutations are often shown in lex order, but rev colex order has the advantage that it can be used to order an infinite number of permutations.
This above all means the finitary symmetric group on , which is the union of all finite symmetric groups, and thus contains each finitary permutation of .
Its n-th element (counted from 0) is found in row n of Sloane'sA055089.

This makes it possible to assign a number to a permutation without knowing how many fixed points it has, by interpreting its left inversion count as a (little-endian) factorial number.

The reverse colex order of the permutations corresponds to the colex order of these left inversion counts, as seen in the following illustrations.

The 24 permutations of
that have a complete 5-cycle
The 12 even permutations of
(the ones that are not green or orange in the table on the right)
The 24 permutations of in RevCoLex order (reflected factorial numbers shown below)

This is the top left submatrix of all bigger tables of this kind (compare this one).
Some permutations of
Here the blue patterns are not horizontally symmetric.


Infinite orderings of integer partitions and set partitions can be defined using CoLex ordering.

Partitions of 10

The binary vectors are in CoLex order and correspond to the increasing sequence Sloane'sA194602 = 0,1,3,5,7,11,15...
Partitions of a 4-set

The binary matrix in CoLex order
is the top left corner of the right matrix in the box below.

Walsh functions[edit]

The rows of binary Walsh matrices in Lex and CoLex order give symmetric matrices.
For normal Walsh matrices (with 1 and −1 instead of 0 and 1) RevLex and RevCoLex would give this result.

The arguments (read like binary vectors) are in Lex order.
Natural order
The arguments (r.l.b.v.) are in CoLex order.


Subsets in Lex order
Subsets ordered with the same pattern, but reflected
(This order does not seem to have a name.)

The following Python code shows the 16 subsets of the set {a,b,c,d} in lexicographic order.
The second part shows that Lex order is the same as binary CoLex order, when the subsets themselves are reversed.

>>> asc =  ['', 'a', 'b', 'ab', 'c', 'ac', 'bc', 'abc', 'd', 'ad', 'bd', 'abd', 'cd', 'acd', 'bcd', 'abcd']
>>> sorted(asc)
['', 'a', 'ab', 'abc', 'abcd', 'abd', 'ac', 'acd', 'ad', 'b', 'bc', 'bcd', 'bd', 'c', 'cd', 'd']

>>> desc = ['', 'a', 'b', 'ba', 'c', 'ca', 'cb', 'cba', 'd', 'da', 'db', 'dba', 'dc', 'dca', 'dcb', 'dcba']
>>> print desc == sorted(desc)
Subsets (r.l.b.v.) in CoLex order,
also the Lex order of the reversed subsets
Subsets (r.l.b.v.) in Lex order