# Notation for a matrix with same vector in columns

Is there a notation for a matrix which columns are the same vector?

$$x = \left[\begin{array}{ccc}x_1 & x_2 & x_3\end{array}\right]^{T} \\ y = \left[\begin{array}{ccc}x & x & x\end{array}\right]$$

Is there a proper notation for $y$?

• Maybe $[1,1,1]\otimes x$ ? I think maybe it is $\sum_i e_i\otimes x$. I don't have full insight in how the kronecker product relates to the tensor product, but i think both can be see as "vector times vector".
– Emil
Aug 29 '18 at 7:18
• $Y=x1^T$ is often useful. Here $1$ is a column vector, whose elements are all equal to one.
– greg
Aug 29 '18 at 18:15

The notation you have for $y$ is fine. It's common block matrix notation.

Sometimes I've seen it wrote like this $$\left(\begin{matrix} |&|&|\\x&x&x\\|&|&|\end{matrix}\right)$$ The form you suggested doesn't seems right to me, but I could be wrong!