MathJax Equation Demo

by keithphw

HTML

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I'm trying to achieve the following equation,
\[\beta_V = \frac{D}{V}\beta_D + \frac{E}{V}\beta_E\]
but with up or down arrows centered above the terms.
<br/><br/>
<b>Update:</b> thanks to Frederic Wang and William Hammond for their pointers. Here is the new best-case version. The only problem is that there's too much space between the fraction and beta terms.
\[\begin{array}{c c c c c c c}
\cdot    & & \uparrow    & \uparrow & & \downarrow  & \uparrow \\
\beta_V  &=& \frac{D}{V} & \beta_D  &+& \frac{D}{E} & \beta_E \\
\end{array}\]

Using Frederic's advice, it sounds as though the only way to adjust the column spacing is to provide my own custom mathjax configuratiuon file which changes the column spacing for all 'array' environments in a latex document. This has the disadvantage that it affects all array environments. 
Or I could make my own new environment such as array_no_space, but this will result in non-portable latex code that only works on my web site.
<br/>
It would be great if mathjax supported a way to change the column spacing in a portable way, such as specifying the following which I beleive works in most latex editors:
\[
{\setlength{\arraycolsep}{1pt}
    \begin{array}{c c c c c c c}
    \cdot    & & \uparrow    & \uparrow & & \downarrow  & \uparrow \\
    \beta_V  &=& \frac{D}{V} & \beta_D  &+& \frac{D}{E} & \beta_E \\
    \end{array}
}
\]

<br/><br/>
Older methods:
<br/>
These are my attempts using different methods, in order of preference.<br/>
Method 1. Problem is that arrows are left-aligned rather than center-aligned. Otherwise this is ideal:

\[\begin{alignat*}{9}
&&&\cdot      &&   &&\uparrow    &&\uparrow   &&\downarrow &&\uparrow \\
&&&\beta_V &&= &&\frac{D}{V}&&\beta_D + &&\frac{E}{V}   &&\beta_E...