[Tex/LaTex] Change size of font of subscript of underbrace in align environment

alignfontsizesubscripts

I would like to have a bigger subscript S under the underbrace in the following equation and appreciate your help.

\begin{align}
K&=\frac{E}{(1+i)^r}\cdot \underbrace{\left (1+\frac{1}{(1+i)^{g}}+\frac{1}{(1+i)^{2\cdot g}}+\frac{1}{(1+i)^{3\cdot g}}+...+\frac{1}{(1+i)^{(n-1)\cdot g}}+\frac{1}{(1+i)^{n\cdot g}}\right )}_S\\
\end{align}


Just use _{\textstyle S}, perhaps adding \mathstrut to give a bit more room.

Here a couple of examples, the second uses multlined (which requires mathtools) for splitting the big underbraced object into two lines:

\documentclass{article}
\usepackage{amsmath,mathtools}

\begin{document}

$$K=\frac{E}{(1+i)^r}\cdot \underbrace{\left( 1+\frac{1}{(1+i)^{g}}+\frac{1}{(1+i)^{2\cdot g}} +\frac{1}{(1+i)^{3\cdot g}} +\dots +\frac{1}{(1+i)^{(n-1)\cdot g}}+\frac{1}{(1+i)^{n\cdot g}}\right ) }_{\textstyle S\mathstrut}$$

$$K=\frac{E}{(1+i)^r}\cdot \underbrace{ \begin{multlined}[t] \biggl( 1+\frac{1}{(1+i)^{g}}+\frac{1}{(1+i)^{2\cdot g}} +\frac{1}{(1+i)^{3\cdot g}} +\dots \\ +\frac{1}{(1+i)^{(n-1)\cdot g}}+\frac{1}{(1+i)^{n\cdot g}} \biggr) \end{multlined} }_{\textstyle S\mathstrut}$$

\end{document}


Notes: never use align for a single line display; avoid ... and prefer \dots.

I'd also write 2g, 3g and so on, without \cdot, unless this is not ordinary multiples and \cdot denotes a special action.