Matrix of ones

In mathematics, a matrix of ones or all-ones matrix is a matrix with every entry equal to one. For example:

$J_2=\left[\begin{array}{cc}1& 1\\ 1& 1\end{array}\right],J_3=\left[\begin{array}{ccc}1& 1& 1\\ 1& 1& 1\\ 1& 1& 1\end{array}\right],J_{2,5}=\left[\begin{array}{ccccc}1& 1& 1& 1& 1\\ 1& 1& 1& 1& 1\end{array}\right],J_{1,2}=\left[\begin{array}{cc}1& 1\end{array}\right].$

Some sources call the all-ones matrix the unit matrix, but that term may also refer to the identity matrix, a different type of matrix.

A vector of ones or all-ones vector is matrix of ones having row or column form; it should not be confused with unit vectors.

Properties

For an n × n matrix of ones J, the following properties hold:

• The trace of J equals n, and the determinant equals 0 for n ≥ 2, but equals 1 if n = 1.
• The characteristic polynomial of J is $(x-n)x^{n-1}$.
• The minimal polynomial of J is $x^2-nx$.
• The rank of J is 1 and the eigenvalues are n with multiplicity 1 and 0 with multiplicity n − 1.
• $J^k=n^{k-1}J$ for $k=1,2,\dots .$
• J is the neutral element of the Hadamard product.

When J is considered as a matrix over the real numbers, the following additional properties hold:

• J is positive semi-definite matrix.
• The matrix $\frac{1}{n}J$ is idempotent.
• The matrix exponential of J is $\exp (\mu J)=I+\frac{e^{\mu n}-1}{n}J$

Applications

The all-ones matrix arises in the mathematical field of combinatorics, particularly involving the application of algebraic methods to graph theory. For example, if A is the adjacency matrix of an n-vertex undirected graph G, and J is the all-ones matrix of the same dimension, then G is a regular graph if and only if AJ = JA. As a second example, the matrix appears in some linear-algebraic proofs of Cayley's formula, which gives the number of spanning trees of a complete graph, using the matrix tree theorem.

The logical square roots of a matrix of ones, logical matrices whose square is a matrix of ones, can be used to characterize the central groupoids. Central groupoids are algebraic structures that obey the identity $(a\cdot b)\cdot (b\cdot c)=b$. Finite central groupoids have a square number of elements, and the corresponding logical matrices exist only for those dimensions.

See also

• Zero matrix, a matrix where all entries are zero
• Single-entry matrix