All-ones matrix ($J$)

The all-ones matrix $J=\mathbf{e}{\mathbf{e}}^T$ has every entry equal to one, where $\mathbf{e}=(1,\dots ,1{)}^T$. On ${\mathbb{R}}^n$, the matrix $J/n$ is the orthogonal projection matrix onto $\mathrm{span}\{\mathbf{e}\}$.