Walsh permutation
The term Walsh permutation was chosen by the author for permutations that permute Walsh functions into other Walsh functions.
A Walsh permutation of degree n has length 2^{n}, and corresponds to an n×n matrix in the general linear group of degree n over the finite field.
This matrix will sometimes be called compression matrix, and its expression as a vector of n integers will be called compression vector.
There are A002884(n) Walsh permutations of degree n.
Not all vectors with different elements correspond to Walsh permutations, as the following example shows:
No Walsh permutation wp(1,2,3,4)  


Notation warning
[edit  edit source]This article and its subpages currently use two different ways to name and display Walsh permutations. In older files the direction of the compression vector is horizontal, and the permutation is vertical. In new files both compression vector and permutation are horizontal. The old files are gradually replaced. 3bit Walsh permutation already uses new files.
When a simple permutation of n elements is applied on the binary digits of numbers from 0 to 2^{n}1 the result is a permutation of the numbers from 0 to 2^{n}1.
The example on the right corresponds to the simple permutation that swaps the first two and the last two digits of the 4bit binary numbers.
Probably the most important bit permutation is the bitreversal 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.
See also Algebraic normal form.
The permutation that changes the natural ordered into the sequency ordered Walsh matrix is the product of the Gray code permutation and the bitreversal permutation.
Each bent function corresponds to a Walsh permutation.
This can be seen when all rows of its sec matrix are XORed with the function itself (second step in the example on the right).
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 2^{n}1 nbit Walsh permutations with horizontally striped inversion sets (like the left one of the examples).
The pattern of the stripes is that of Walsh functions.
2bit
[edit  edit source]The A002884(2) = 6 2bit Walsh permutations form general linear group GL(2,2), which is also the symmetric group S_{3}.
3bit
[edit  edit source]The A002884(3) = 168 3bit Walsh permutations correspond to the collineations of the Fano plane.
Compression vectors, Permutations
4bit
[edit  edit source]There are A002884(4) = 20160 4bit Walsh permutations.
Fixed points
[edit  edit source]The fixed points of an nbit Walsh permutation are seals of arity n.