# How can I customize equations numbering in subequations enviroment?

In the figure above, the subequations part of numbering is alphabetical, but I want to customize both the equation and the subequation part to look like this: a-I, a-II, and a-III. Here is the source code:

\documentclass{book}

\usepackage{amsmath}
\usepackage{amssymb, amsfonts}

\usepackage{subfloat}

\begin{document}

\begin{subequations}

\begin{align}
I_{sc} & = I_{sc,ref} & \hspace{-30 mm} \bigg[  1 + \frac{\alpha}{100} (T_{op}-T_{ref}) \biggl]
\vspace{2 mm}
\label{eq:I_sc_trans_STC2}
\\
%
V_{oc} & = V_{oc,ref} & \hspace{-30 mm} \bigg[  1 + \frac{\beta}{100} (T_{op}-T_{ref}) \biggl]
\vspace{2 mm}
\label{eq:V_oc_trans_STC}
\\
%
P_{mp} & = P_{mp,ref} & \hspace{-30 mm} \bigg[  1 + \frac{\gamma}{100} (T_{op}-T_{ref}) \biggl]
\label{eq:P_mp_trans_STC}
\end{align}

\end{subequations}

\end{document}

• The env is not customizable. But it is simple, find the code for it in amsmath.sty and make your own version in your preamble – daleif Jul 7 '17 at 5:37
• I removed the cleverref tag because the problem does not seem to involve the cleverref package at all (it's not loaded in the MWE). Feel free to rollback the edit if I am mistaken. – Troy Jul 7 '17 at 6:24

You can put

\renewcommand{\theequation}{\alph{parentequation}--\Roman{equation}}


just after \begin{subequations}.

In the following MWE I've used alignat instead of align to avoid using text commands for horizontal space, and I've adjusted vertical space between equations using things like \\[2mm] instead of \vspace{2 mm}\\. Also text subscripts look better with \text.

\documentclass{book}

\usepackage{amsmath}
\usepackage{amssymb, amsfonts}

\begin{document}

\begin{subequations}
\renewcommand{\theequation}{\alph{parentequation}--\Roman{equation}}

\begin{alignat}{2}
I_{\text{sc}} & = I_{\text{sc},\text{ref}} & \bigg[  1 + \frac{\alpha}{100} (T_{\text{op}}-T_{\text{ref}}) \biggl]
\label{eq:I_sc_trans_STC2}
\\[2mm]
%
V_{\text{oc}} & = V_{\text{oc},\text{ref}} & \bigg[  1 + \frac{\beta}{100} (T_{\text{op}}-T_{\text{ref}}) \biggl]
\label{eq:V_oc_trans_STC}
\\[2mm]
%
P_{\text{mp}} & = P_{\text{mp},\text{ref}} & \bigg[  1 + \frac{\gamma}{100} (T_{\text{op}}-T_{\text{ref}}) \biggl]
\label{eq:P_mp_trans_STC}
\end{alignat}

\end{subequations}

\end{document}


If you need this behavior globally, i.e. \alpha numbering for all equations and \Roman numbering for all subequations, add the following lines in your preamble:

\usepackage{etoolbox}
\patchcmd{\subequations}{\alph{equation}}{--\Roman{equation}}{}{}

\renewcommand{\theequation}{\alph{equation}}


\renewcommand{\theequation}{\alph{parentequation}--\Roman{equation}}

• Sorry, for my typo in the question. I did not mean changing the numbering to a-I, b-II, and c-III, but rather to a-I, a-II, and a-III meaning that is a set of equations a and its subequations are given in roman letters – Al-Motasem Aldaoudeyeh Jul 7 '17 at 6:07
• @Al-MotasemAldaoudeyeh I've rejected your edit because you've deleted some important improvements to the code (see \text). – Arzigoglu Jul 7 '17 at 6:47