What is an elegant way of indicating repeating elements in a vector?

Question

Consider a vector that is made up of N times the same element, stacked next to each other. I am having a hard time coming up with an elegant mathematical notation for indicating the repetitiveness of the elements, and the how often they repeat.

The best I could come up with is:

\documentclass{article}

\usepackage{mathtools}

\begin{document}
\[
    \underbrace{%
        \begin{pmatrix} 
            \Delta t \mathbf{a}_b &  \Delta t \mathbf{a}_b & \cdots & \Delta t
            \mathbf{a}_b 
        \end{pmatrix}%
     }_{N \times (\Delta t \mathbf{a}_b)} 
\]
\end{document}

Can you propose a way to make this clearer in LaTeX?

Answer 1

This is very similar to another answer.

Since the duplication of elements already suggest repetition, I would merely highlight the number of elements/items in the vector:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}

\[
  \bigl( \underbrace{%
    \Delta t \mathbf{a}_b \quad \Delta t \mathbf{a}_b \quad \cdots \quad \Delta t \mathbf{a}_b
  }_{\text{$N$~elements}} \bigr)
\]

\end{document}

Answer 2

I would simply write

\documentclass{article}

\usepackage{mathtools}

\begin{document}
\[
    \underbrace{%
        \begin{pmatrix}
            \Delta t \mathbf{a}_b &  \Delta t \mathbf{a}_b & \cdots & \Delta t
            \mathbf{a}_b
        \end{pmatrix}%
     }_{N \text{ times}}
\]
\end{document} 

Answer 3

What you have might work, but here is an alternative (especially if you need it in \textstyle):

\documentclass{article}
\begin{document}
$[{(\Delta t\, \mathbf{a}_b)}_{\times N}] $
\end{document}

Answer 4

Not necessarily implying that my proposition is superior, I think dot notation is old. You can also add a subscript to ones vector for size info or remove \cdot.

\documentclass{article}
\usepackage{mathtools,bbm}%Use your favorite font for bboard number
\begin{document}
\noindent
If someone defines $\mathbbm{1}$ as all-ones vector,
\[ \Delta t \cdot a_b \cdot \mathbbm{1}^T \]
\end{document}

Answer 5

I would like to propose yet another way:

\documentclass{article}

\usepackage{mathtools}

\begin{document}
% Row vector
\[
    \begin{pmatrix} 
        \Delta t \mathbf{a}_b  \stackrel{\times N}{\cdots\cdots}
    \end{pmatrix}
\]
% Column vector
\[
    \begin{pmatrix} 
        \Delta t \mathbf{a}_b^T\\
          \stackrel{\vdots}{\scriptstyle\times N}
    \end{pmatrix}
\]
\end{document}

There is an issue with the spacing for column vectors, though.