Weighing matrix

In mathematics, a weighing matrix W of order n and weight w is an n × n (0,1,-1)-matrix such that $WW^{T}=wI_n$, where $W^T$ is the transpose of $W$ and $I_n$ is the identity matrix of order $n$.

For convenience, a weighing matrix of order n and weight w is often denoted by W(n,w). A W(n,n) is a Hadamard matrix and a W(n,n-1) is equivalent to a conference matrix.

Properties

Some properties are immediate from the definition. If W is a W(n,w), then:

• The rows of W are pairwise orthogonal (that is, every pair of rows you pick from W will be orthogonal). Similarly, the columns are pairwise orthogonal.
• Each row and each column of W has exactly w non-zero elements.
• $W^{T}W=wI$, since the definition means that $W^{-1} = w^{-1}W^{T}$, where $W^{-1}$ is the inverse of $W$.
• $\operatorname{det}(W)=\pm w^{n/2}$ where $\operatorname{det}(W)$ is the determinant of $W$.

Examples

Note that when weighing matrices are displayed, the symbol $-$ is used to represent -1. Here are two examples:

This is a W(2,2):

$\begin{pmatrix}1 & 1 \\ 1 & -\end{pmatrix}$

This is a W(7,4):

$\begin{pmatrix} 1 & 1 & 1 & 1 & 0 & 0 & 0 \\ 1 & - & 0 & 0 & 1 & 1 & 0 \\ 1 & 0 & - & 0 & - & 0 & 1 \\ 1 & 0 & 0 & - & 0 & - & - \\ 0 & 1 & - & 0 & 0 & 1 & - \\ 0 & 1 & 0 & - & 1 & 0 & 1 \\ 0 & 0 & 1 & - & - & 1 & 0 \end{pmatrix}$

Equivalence

Two weighing matrices are considered to be equivalent if one can be obtained from the other by a series of permutations and negations of the rows and columns of the matrix. The classification of weighing matrices is complete for cases where w ≤ 5 as well as all cases where n ≤ 15 are also completed. However, very little has been done beyond this with exception to classifying circulant weighing matrices.

Open Questions

There are many open questions about weighing matrices. The main question about weighing matrices is their existence: for which values of n and w does there exist a W(n,w)? A great deal about this is unknown. An equally important but often overlooked question about weighing matrices is their enumeration: for a given n and w, how many W(n,w)'s are there?