Font size inside subequations environment!

Question

How I can change the font size inside the subequations environment? See below:

\begin{equation}
    \textstyle
    \xi_{1} = A + B + C;
\end{equation}

The command \textstyle works fine inside the equation environment, but inside the subequations environment (see below, again) does not works.

\begin{subequations}
    \textstyle
    \begin{eqnarray}
      \xi_{1} = A + B + C \\
      \xi_{2} = E + F + G \\
      \xi_{3} = H + I + J
    \end{eqnarray}
\end{subequations}

I give errors:

"Missing $ inserted.", "Missing \endgroup inserted.", "You can't use \halign in math mode", etc.

If I put \textstyle inside eqnarray it works, but then I need put in all equations.

Please, how I can change the math font size inside the subequations environment?

Answer 1

Define an alignts environment that's the same as align, but uses \textstyle throughout.

The problem with long alignments is solved by issuing \allowdisplaybreaks. I tested with 216 equations without any problem. Don't use eqnarray under any circumstances.

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}

\makeatletter
\let\alignts@preamble\align@preamble
\patchcmd{\alignts@preamble}{\displaystyle}{\textstyle}{}{}
\patchcmd{\alignts@preamble}{\displaystyle}{\textstyle}{}{}

\def\alignts{\let\align@preamble\alignts@preamble\start@align\@ne\st@rredfalse\m@ne}
\let\endalignts\endalign
\makeatother

\allowdisplaybreaks

\begin{document}
\begin{alignts}
\xi_{1} &= \sum_{i=1}^n a_i \\
\xi_{2} &= \int_a^b f(x)\,dx \\
\xi_{3} &= \bigvee_i v_i
\end{alignts}
\end{document}

Embed this in a subequations environment, the result will be the same as subequations only changes the equation counter and has no influence on the typesetting of the equations.

If you insist in using eqnarray, it's just the same:

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}

\makeatletter
\let\eqnarrayts\eqnarray
\patchcmd{\eqnarrayts}{\displaystyle}{\textstyle}{}{}
\patchcmd{\eqnarrayts}{\displaystyle}{\textstyle}{}{}
\let\endeqnarrayts\endeqnarray
\makeatother

\allowdisplaybreaks

\begin{document}

\begin{eqnarrayts}
\xi_{1} = \sum_{i=1}^n a_i \\
\xi_{2} = \int_a^b f(x)\,dx \\
\xi_{3} = \bigvee_i v_i
\end{eqnarrayts}

\end{document}