Wsrtjtyk

I have an expression with an underbrace, where the parenthesis and a second partial devirative gets a little to big.

Here is the code:

begin{fleqn}

begin{align*}

    frac{1}{v_G} =  frac{1}{v_{ph}} cdot left(frac{1 - frac{lambda}{n} frac{partial n}{partial lambda}}{1 - left(frac{lambda}{n} right)^2 underbrace{frac{partial^2 n}{partial lambda^2}}_{=0}} right) = frac{1}{v_{ph}} cdot left( 1 - frac{lambda}{n} frac{partial n}{partial lambda} right) 

end{align*}

end{fleqn}

and here the result:

How can I adjust the code so that the second order partial derivative is as big as the term over the fraction line?

I would simply omit those redundant parentheses. And most certainly fix the size of the underbrace object.

It's also possible (but requires a phantom) to have the right size for the parentheses:

documentclass{article}

usepackage{amsmath}



makeatletter

newcommand{fixedunderbrace}[2]{{mathpalettefixed@underbrace{{#1}{#2}}}}

newcommand{fixed@underbrace}[2]{fixed@@underbrace#1#2}

newcommand{fixed@@underbrace}[3]{underbrace{#1#2}_{#3}}

makeatother



begin{document}



begin{equation*}

frac{1}{v_G} =

frac{1}{v_{ph}} cdot 

  frac{1 - frac{lambda}{n} frac{partial n}{partial lambda}}

       {1 - left(frac{lambda}{n} right)^2

        fixedunderbrace{frac{partial^2 n}{partial lambda^2}}{=0}}

= frac{1}{v_{ph}} cdot 

  left( 1 - frac{lambda}{n} frac{partial n}{partial lambda} right) 

end{equation*}



begin{equation*}

frac{1}{v_G} =

frac{1}{v_{ph}} cdot 

  left(

  frac{1 - frac{lambda}{n} frac{partial n}{partial lambda}}

       {1 - left(frac{lambda}{n} right)^2

        smash[b]{fixedunderbrace{frac{partial^2 n}{partial lambda^2}}{=0}}}

  right)

  vphantom{

    frac{1 - frac{lambda}{n} frac{partial n}{partial lambda}}

       {1 - left(frac{lambda}{n} right)^2

        fixedunderbrace{frac{partial^2 n}{partial lambda^2}}{=0}}

  }

= frac{1}{v_{ph}} cdot 

  left( 1 - frac{lambda}{n} frac{partial n}{partial lambda} right) 

end{equation*}



end{document}

Some suggestions (the first two of which have already been made by @egreg):

• Omit the large parentheses entirely -- they seem to satisfy no useful purpose.




• Use tfrac (or textstylefrac) instead of frac in the first argument of underbrace. (By default, the first argument of underbrace is processed in display-style math mode. However, the remainder of the denominator is processed in text style, not display style. That's why it's necessary to write tfrac instead of just frac.)




• Omit all cdot directives. They achieve little but clutter up the appearance of the equation.




• For better horizontal spacing as well as for more typographically-appropriate sizing of the parentheses in the final part of the equation, use Bigl( and Bigr) instead of left( and right).

documentclass{article}

usepackage{amsmath} % for "tfrac" macro

begin{document}

[

frac{1}{v_G} 

= frac{1}{v_{ph}}  

frac{1 - frac{lambda}{n} frac{partial n}{partial lambda}}{%

      1 - bigl(frac{lambda}{n} bigr)^{!2} !

underbrace{tfrac{partial^2 n}{partiallambda^2}}_{=0}}

= frac{1}{v_{ph}} Bigl( 1 - frac{lambda}{n} 

    frac{partial n}{partial lambda} Bigr)

]

end{document}

You can either use the cfrac command to have all fractions in display style, or tfrac for the underbraced partial derivative. I don't like such a discrepancy between these two sizes in the same formula, so I suggest using the medium size commands from nccmath (~80% of displaystyle). I give an example of both, with some spacing improvements, and in the second example, the use ofthe esdiff package to simpify typing of partial derivatives:

documentclass{article}



usepackage{amsmath, nccmath}

usepackage{esdiff}

usepackage{bigstrut}



begin{document}



begin{fleqn}

begin{align*}

    frac{1}{v_G} = frac{1}{v_{ph}} cdot left(frac{1 - cfrac{lambda}{nbigstrut[b]} cfrac{partial n}{partial lambda}}{1 - biggl(cfrac{lambda}{n} biggr)^{mkern-5mu 2} smash[b]{underbrace{cfrac{partial^2 n}{partial lambda^2}}_{=0}}} right) = frac{1}{v_{ph}} cdot left( 1 - frac{lambda}{n} frac{partial n}{partial lambda} right)

end{align*}

bigskip



begin{align*}

    frac{1}{v_G} = frac{1}{v_{ph}} cdot left(frac{1 - medmath{frac{lambda}{nbigstrut[b]} diffp{n}{lambda}}}{1 - medmath{ left(frac{lambda}{n} right)^{mkern-5mu 2} smash[b]{underbrace{diffp[2]{n}{lambda}}_{=0}}}} right) = frac{1}{v_{ph}} cdot left( 1 - frac{lambda}{n} diffp{n}{lambda} right)

end{align*}

end{fleqn}



end{document}