When the rows of a Walsh matrix are permuted, the columns of the resulting matrix are usually no Walsh functions anymore.
When the permutation is a Walsh permutation, they are.
Walsh permutations of 2n elements can be represented by an n-element compression vector
of integers referring to the numbering of Walsh functions (displayed in orange in the images).
It corresponds to the n×n compression matrix. It's columns show the elements of the compression vector in binary representation.
The multiplication of the compression matrices (with F2 operations) corresponds to the composition of the permutations.
As the compression matrices are the elements of the general linear group GL(n,2) the Walsh permutations form a group isomorphic to GL(n,2).
|Another example: Gray * bitrev|
This product permutes the natural ordered Hadamard matrix (Walsh matrix) in the sequency ordered Hadamard matrix:
In the latter one the number of sign changes per row is consecutive.
Not all vectors with different elements correspond to Walsh permutations, as the following example shows:
|No Walsh permutation wp(1,2,3,4)|
When a simple permutation of n elements is applied on the binary digits of numbers from 0 to 2n-1 the result is a permutation of the numbers from 0 to 2n-1.
The example on the right corresponds to the simple permutation that swaps the first two and the last two digits of the 4-bit binary numbers.
Probably the most important bit permutation is the bit-reversal permutation.
The rows of each nimber multiplication table are Walsh permutations (except row 0).
They are not only closed under multiplication (function composition), but even under addition (bitwise XOR).
Powers of the Gray code permutation have very simple compression matrices and vectors.
In each vector all entries follow from the first one, and the first entries follow from the rows of the Sierpinski triangle.
There are 24*9=216 Walsh permutations that correspond to magic squares of order 4.
One my say that only 6 of them are essentially different.
Some inversion sets of Walsh permutations are very regular.
E.g. there are 2n-1 n-bit Walsh permutations with horizontally striped inversion sets (like the left one of the examples). The pattern of the stripes is that of Walsh functions.