Large Blackboard bold R operator with text underneath

Question

How do I typeset a large blackboard bold R (size of a \sum symbol) as an operator in an equation? See image. I can get the text to display underneath with \mathop{}, but I can't make the R big. \displaystyle doesn't make the symbol any bigger.

I can get a solution of sorts in XeLaTeX, but I would prefer a LaTeX solution.

This is the XeLaTeX solution

\documentclass[a4paper]{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\begin{bmatrix} x \\ y \\ z \end{bmatrix}_{\substack{\text{some text}\\\text{other text}}}
= 
\sum_x\mathop{\text{\huge ℝ}}_{\text{sys1}\rightarrow\text{sys2}}
\begin{bmatrix} X \\ Y \\ Z \end{bmatrix}_{\substack{\text{more text}\\\text{also text}}}
\end{equation*}
\end{document}

As you can see, I've copy-pasted the unicode character and made it \huge, and used \mathop to put the text underneath. It's not an elegant solution, it's not very portable since it depends on the text font instead of a math font and it doesn't center the R on the '=' properly (compare the sum).

What is a better way to do this? Preferably in LaTeX.

Answer 1

This will produce symbols with the same vertical size as \sum in any style and with any font size; any math alphabet can be used.

For a commonly used symbol, define a specific command, as I did for \Rop.

\documentclass{article}
\usepackage{amsmath,graphicx,amssymb}
\usepackage{dsfont} % for the example


\makeatletter
\DeclareRobustCommand{\letterop}[1]{\mathop{\mathpalette\letterop@do{#1}}}
\newcommand\letterop@do[2]{%
  \vcenter{%
    \sbox{\z@}{$\m@th#1\sum$}%
    \dimen@=\ht\z@ \advance\dimen@\dp\z@
    \sbox{\z@}{$\m@th#1#2$}\dp\z@=\z@ % kill the depth
    \hbox{\resizebox{!}{\dimen@}{\unhbox\z@}}%
  }%
}
\makeatother

\newcommand{\Rop}{\letterop{\mathds{R}}}

\begin{document}

\begin{equation*}
\begin{bmatrix} x \\ y \\ z \end{bmatrix}_{\substack{\text{some text}\\\text{other text}}}
= 
\sum_x\Rop_{\text{sys1}\rightarrow\text{sys2}}
\begin{bmatrix} X \\ Y \\ Z \end{bmatrix}_{\substack{\text{more text}\\\text{also text}}}
\end{equation*}

\section{In section title $\sum_x\Rop_{\text{sys1}\rightarrow\text{sys2}}$}

Textstyle: \qquad $\sum_x\Rop_{\text{sys1}\rightarrow\text{sys2}}$

Scriptstyle: \qquad $\scriptstyle
\sum_x\Rop_{\text{sys1}\rightarrow\text{sys2}}$

Scriptscriptstyle: \quad $\scriptscriptstyle
\sum_x\Rop_{\text{sys1}\rightarrow\text{sys2}}$

\[ \letterop{\mathrm{S}}_{i=1}^n \letterop{\mathbb{R}}_{i=1}^n \]

\end{document}

Answer 2

This does the job nicely I think (thanks karlkoeller for suggesting improvements):

\documentclass[a4paper]{article}

\pagestyle{empty}

\usepackage{amsmath,amsfonts,relsize,graphicx,dsfont}

\makeatletter
\newcommand*\smallR{\mathds{R}} % <-- this is your symbol
\newcommand*\yo@bigR[2]{\text{#2\raisebox{-#1ex}{$\smallR$}}}
\DeclareRobustCommand*\bigR{\mathop{\mathchoice
  {\yo@bigR{0.5}{\larger\larger\larger\larger}} % displaystyle
  {\yo@bigR{0.4}{\larger\larger}} % textstyle
  {\yo@bigR{0.3}{\larger\larger}} % scriptstyle
  {\text{\scalebox{1.4}{\raisebox{-0.3ex}{$\smallR$}}}} % scriptscriptstyle
  }\displaylimits}
\makeatother

\begin{document}
\begin{equation*}
\begin{bmatrix} x \\ y \\ z \end{bmatrix}_{\substack{\text{some text}\\\text{other text}}}
= 
\sum_x\bigR_{\text{sys1}\rightarrow\text{sys2}}
\begin{bmatrix} X \\ Y \\ Z \end{bmatrix}_{\substack{\text{more text}\\\text{also text}}}
\end{equation*}

\centering

\section{In section title $\sum_x\bigR_{\text{sys1}\rightarrow\text{sys2}}$}

Textstyle: \qquad $\sum_x\bigR_{\text{sys1}\rightarrow\text{sys2}}$

Scriptstyle: \qquad $\scriptstyle
\sum_x\bigR_{\text{sys1}\rightarrow\text{sys2}}$

Scriptscriptstyle: \quad $\scriptscriptstyle
\sum_x\bigR_{\text{sys1}\rightarrow\text{sys2}}$

\end{document}

It would somehow work by just using:

\newcommand\bigR{\mathop{\text{\LARGE\raisebox{-0.5ex}{$\mathds{R}$}}}\limits}

The rest of the mess above is to make it work in any environment and possibly in any font size.

Answer 3

You can use this:

\newcommand*{\Scale}[2][4]{\scalebox{#1}{$#2$}}%
\newcommand*{\Resize}[2]{\resizebox{#1}{!}{$#2$}}%
\newcommand{\MathOp}[5]{\displaystyle{\mathop{\Scale[#1]{#2}}_{#3}^{#4}#5}}

and thy with

\MathOp{1.5}{\mathbb R}{pedex}{apex}{something}

Good by