I need to show that $\overline{\mathbf{J}}=\frac{1}{n}\mathbf{J}=\frac{1}{n}\mathbf{1}\mathbf{1}^{\prime}$ is idempotent, being $\mathbf{1}$ a unit vector of $n \times 1$
By opperating, I've got that $\overline{\mathbf{J}}=\frac{1}{n}\mathbf{J}=\frac{1}{n}\mathbf{1}\mathbf{1}^{\prime}=\frac{1}{n^2}{\mathbf{J}}$ and I don't know that's next or how to argue.