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Matrix of ones

${\displaystyle J_{2}={\begin{pmatrix}1&1\\1&1\end{pmatrix}};\quad J_{3}={\begin{pmatrix}1&1&1\\1&1&1\\1&1&1\end{pmatrix}};\quad J_{2,5}={\begin{pmatrix}1&1&1&1&1\\1&1&1&1&1\end{pmatrix}};\quad J_{1,2}={\begin{pmatrix}1&1\end{pmatrix}}.\quad }$

Some sources call the all-ones matrix the unit matrix,^[2] but that term may also refer to the identity matrix, a different type of matrix.

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

Properties[edit]

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

• The traceofJ equals n,^[3] and the determinant equals 0 for n ≥ 2, but equals 1 if n = 1.
• The characteristic polynomialofJis${\displaystyle (x-n)x^{n-1}}$.
• The minimal polynomialofJis${\displaystyle x^{2}-nx}$.
• The rankofJ is 1 and the eigenvalues are n with multiplicity 1 and 0 with multiplicity n − 1.^[4]
• ${\displaystyle J^{k}=n^{k-1}J}$ for ${\displaystyle k=1,2,\ldots .}$^[5]
• J is the neutral element of the Hadamard product.^[6]

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

• Jispositive semi-definite matrix.
• The matrix ${\displaystyle {\tfrac {1}{n}}J}$isidempotent.^[5]
• The matrix exponentialofJis${\displaystyle \exp(\mu J)=I+{\frac {e^{\mu n}-1}{n}}J}$

Applications[edit]

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.^[7] 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.

See also[edit]

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

References[edit]

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