Using \limits in a customized \overset for superscript together with \sideset for subscript

Question

I am trying to create a notation for a certain space. I am using a customized \overset in mrf's answer in this thread to suppress the extra vertical space. I still want the subscript on the right side, so I thought \sideset might come in handy since I used it before as in this thread. The \overset is redefined as follows in this example:

\documentclass{minimal}
\usepackage{amsmath}

\renewcommand{\overset}[2]{%
\mathop{#2}\limits^{\vbox to -.1ex{%
\kern -0.4ex\hbox{$\scriptstyle #1$}\vss}}}

\begin{document}
$$
\sideset{}{_0}\overset{\circ}{\mathbf{H}}(\mathbf{curl};\Omega)
$$
\end{document}

But unfortunately LaTeX gave me an error:

Argument of \overset has an extra } \sideset{}{_0}\overset.

A compromise can be adding the subscript inside \overset by \overset{\circ}{\mathbf{H}_0}(\mathbf{curl};\Omega), the horizontal alignment of \circ is now to the whole \mathbf{H}_0 and not to \mathbf{H}.

I have tried the amsmath built-in \overset in this example after \sideset and LaTeX stil gives error. I wonder if there is solution that accommodates both.

Answer 1

You are misusing \sideset which can only be applied to big operators such as \sum. Instead just enclose the construction with the circled H in a group and add a subscript to that.

\documentclass{minimal}
\usepackage{amsmath}

\renewcommand{\overset}[2]{%
\mathop{#2}\limits^{\vbox to -.1ex{%
\kern -0.4ex\hbox{$\scriptstyle #1$}\vss}}}

\begin{document}

\begin{equation*}
{\overset{\circ}{\mathbf{H}}}_0(\mathbf{curl};\Omega)
\end{equation*}

\end{document}

As Barbara Beeton points out, your code results in the circled H being vertically centered. If that is not desired then you should add an empty group before the \mathbb{H}:

{\overset{\circ}{{}\mathbf{H}}}_0(\mathbf{curl};\Omega)

This is caused by a percularity/design feature of \mathop, which vertically centers a single symbol, but not multiple ones.