Change size of font of subscript of underbrace in align environment

Question

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}

Answer 1

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}

\begin{equation}
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}
\end{equation}

\begin{equation}
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{equation}

\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.

Answer 2

It does not fit on my page but here you are:

% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}
\usepackage{relsize}

\begin{document}
\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 )}_{\mathlarger{S}}
\end{align}
\end{document}

Maybe better:

\begin{align*}
    K&=\frac{E}{(1+i)^r}\biggl(\underbrace{1+\frac{1}{(1+i)^{g}}+\frac{1}{(1+i)^{2g}}+\frac{1}{(1+i)^{3g}}+\dots+\frac{1}{(1+i)^{(n-1)g}}+\frac{1}{(1+i)^{ng}}}_{\mathlarger{S}}\biggr)
\end{align*}

Feel free to do _{\mathlarger{\mathlarger{\mathlarger{S}}}} and alike.