# Lexicographic and colexicographic order

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).

## Combinations

The sequence of the $\scriptstyle \binom{n}{k}$ k-subsets of $\scriptstyle \{1,...,n\}$ in CoLex order
is the beginning of the infinite sequence of k-subsets of $\scriptstyle \{1,2,3,...\}$ in CoLex order.
This corresponds to the increasing sequence = 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 $\scriptstyle \binom{6}{3} = 20$ 3-subsets of $\scriptstyle \{1,...,6\}$ The 10 3-subsets of $\scriptstyle \{1,...,6\}$ with an even sum of elements The $\scriptstyle \binom{5}{3} = 10$ 3-subsets of $\scriptstyle \{1,...,5\}$ (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 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


## Permutations

Permutations are often shown in Lex order (see here),
but RevCoLex is the order that works for an infinite number of permutations
and corresponds to the CoLex order of the permutations' inversion vectors (shown in red in the following files, except the one on the right).

 The 24 permutations of $\scriptstyle \{1,...,5\}$ that have a complete 5-cycle The 12 even permutations of $\scriptstyle \{1,...,4\}$ (the white permutations in the table on the right) The 24 permutations of $\scriptstyle \{1,...,4\}$ in RevCoLex order This is the top left submatrix of all bigger tables of this kind (compare this one). Here the inversion vectors are shown in g b g r. Some permutations of $\scriptstyle \{1,...,4\}$ Here the blue patterns are not horizontally symmetric.

## Partitions

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 = 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 binary matrix in the file below.

## Walsh functions

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.

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