I have a vector $\mathbf{x} = (x_1,x_2)$, how do I represent the array of such vector?

Is it $\mathbf{x} \in \mathbf{X}$ where $\mathbf{X}$ = $\bigcup _{{1}}^{k}\mathbf{x}^s$ for $s \in (1,..k)$ length of $k$ vectors? Something seems off here!

Also I want to index a vector in array then is this correct way to represent the index ${\mathbf{x}^1}$? and the corresponding dimensional representation of this index shall be $(x_1^1,x_2^1)$?

Is the usage of "hat" appropriate here ? such as "$\hat{x}$"?


1 Answer 1


I'm not sure if by "array of vectors" you intend a single row with the vectors listed, or if you mean a rectangular array with the components as entries or what. But I think the most common thing one would do in a situation like this is to put them in a rectangular matrix and use double indices.

That is, you would have $m$ vectors with $n$ components that look like $(x_{i,1}, \ldots, x_{i, n})$ for $i\in \{1\ldots m\}$, so that the rows of your matrix are the original $n$-tuples.

I have no idea why one would use a hat decorator in this case.

  • $\begingroup$ By array of vectors I mean $[\mathbf{x_i}]$ for $i \in \{1,..,m\}$ so its not rectangular matrix, just vector listed. So will the expanded form be $[(x_{1,i},x_{2,i})]$ $i \in \{1,..,m\}$ ? $\endgroup$
    – user0193
    Sep 30, 2020 at 19:04
  • $\begingroup$ I can think of no better way to write the concatenation as $(x_{1,1},\ldots, x_{1, n},x_{2,1},\ldots x_{2, n},\ldots,x_{m, 1},\ldots x_{m, n})$, or maybe just as $(\mathbf{x_1},\dots,\mathbf{x_m})$ where $\mathbf {x_i}=(x_{i,1},\ldots,x_{i,n})$ $\endgroup$
    – rschwieb
    Sep 30, 2020 at 19:25
  • $\begingroup$ noted. Thanks and also does this makes sense $\mathbf{x} \in \mathbf{X}$ where $\bigcup _{{1}}^{k}\mathbf{x}^s$ for $s i\in \{1\ldots k\}$? $\endgroup$
    – user0193
    Sep 30, 2020 at 19:39
  • 1
    $\begingroup$ @JhonnyS I think using $\bigcup$ is wildly confusing and not helpful. I'm not sure what's out there in terms of a symbol for a concatenation operation, but almost anything is better than $\bigcup$. Even something like $\boxplus$. At any rate you would have to explain that's what you meant, since the notation might not be common. $\endgroup$
    – rschwieb
    Sep 30, 2020 at 20:27
  • 2
    $\begingroup$ @JhonnyS I can see where you're coming from, but for the sanity of the people listening to you, don't call $X$ a set and don't use $\bigcup$. Use something that suggests appending more, like $\oplus$ or $\boxplus$. $\endgroup$
    – rschwieb
    Sep 30, 2020 at 20:36

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