How can I type an expression of a column vector

I want to type a column vector like below:

     [  dx     ]
UI = [  dy     ]
[  dtheta ]


U is the name of this vector, and I is the subscript. Two brackets like the matrix style. dx, dy and dtheta are actually three items with a dot on their head, and theta is a Greek letter.

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For dx, dy and dtheta, you could use \dot{x}, \dot{y} and \dot{\theta}. – Werner Sep 6 '12 at 14:01
If I also want a number of this equation like Ui= [x \\ y \\ theta] (4) (4) is behind the expression indicating its NO. How can I do this? Thanks in advanced. – user18441 Sep 6 '12 at 14:04
Instead of $...$ use $$...$$. – Gonzalo Medina Sep 6 '12 at 14:19

You can use an array to create a vector. See section 3 of the Not so Short Guide to LaTeX for more details.

\documentclass{article}
\begin{document}
$\mathbf{U}_I = \left[ \begin{array}{c} \mathrm{d}x \\ \mathrm{d}y \\ \mathrm{d}\theta \end{array} \right]$
\end{document}


The spacing around between the elements and the brackets can be reduced by replacing {c} with {@{}c@{}}. You could also consider using the bmatrix environment from the amsmath package. Note that $ and $ create an equation that is not numbered. If you need a numbered equation you should use and instead.

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If I also want a number of this equation like Ui= [x \\ y \\ theta] (4) (4) is behind the expression indicating its NO. How can I do this? Thanks in advanced. – user18441 Sep 6 '12 at 13:54
Possibly \begin{array}{@{}c@{}}? – egreg Sep 6 '12 at 13:54
@user18441 --- I have edited my answer. – Ian Thompson Sep 6 '12 at 14:05

Is this what you're after?

$$U_i = \left[ \dot{dx}, \dot{dy}, \dot{d \theta} \right]$$


Well, you can replace the $,$ with any other brace of your taste, e.g. \left(,\right) or \left\{,\right\}. In addition, instead of dx you can have something like \mathrm{d}x to have something similar to derivative.

In addition, if you wish you wish to have column vector, refer to this question and the answers following it.

Edit:

If you want to have a numbered version of the expression, put the expression between the $$, and $$, that is what in the math-mode, inside an equation environment.

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Probably \left[ and \right], not with the additional backslash; but \left and \right are redundant and possibly wrong. – egreg Sep 6 '12 at 13:33
@egreg my bad. I corrected the code – Dror Sep 6 '12 at 13:36
Thanks Dror and Egreg – user18441 Sep 6 '12 at 13:45
@Dror I would remove the \left and \right: it's a bad habit to always add them. In this particular case the output is worse than it would have been without them, just try and look closely at the spacing. – egreg Sep 6 '12 at 13:53
If I also want a number of this equation like Ui= [x \\ y \\ theta] (4) (4) is behind the expression indicating its NO. How can I do this? Thanks in advanced. – user18441 Sep 6 '12 at 14:04