I want to manually label equation number ? For example I want to label this equation 3.1 instead of (1) which automatically pops up. Is it also possible in split environment as well or not ?
\begin{equation} \log \mu_{ijk}= \lambda + \lambda_i ^I + \lambda_j ^J+\lambda_k ^K+\lambda_{ij} ^{IJ}+\lambda_{ik} ^{IK}+\lambda_{jk} ^{JK}+\lambda_{ijk} ^{IJK} \label{eq:3.1} \end{equation} You can use \tag:
\documentclass{article} \usepackage{amsmath} \begin{document} \begin{equation} \log \mu_{ijk}= \lambda + \lambda_i ^I + \lambda_j ^J+\lambda_k ^K+\lambda_{ij} ^{IJ}+\lambda_{ik} ^{IK}+\lambda_{jk} ^{JK}+\lambda_{ijk} ^{IJK} \tag{3.1}\label{eq:3.1} \end{equation} \end{document} 
I guess you are looking for a way to number your equations "within" sections, that is, add the section number before the equation number.
This can be done automatically if you add the line
\numberwithin{equation}{section} in your preamble, without any manual intervention.
MWE:
\documentclass{article} \usepackage{amsmath} \numberwithin{equation}{section} \begin{document} \section{Test} Some text \section{Another Test} Some text \section{An equation} \begin{equation} \log \mu_{ijk}= \lambda + \lambda_i ^I + \lambda_j ^J+\lambda_k ^K+\lambda_{ij} ^{IJ}+\lambda_{ik} ^{IK}+\lambda_{jk} ^{JK}+\lambda_{ijk} ^{IJK} \label{eq:3.1} \end{equation} \end{document} Output:
Personally, I wanted to have "Eq." in front of the equation numbering. I thought you had the same question but then with "equation" instead of "Eq.". Also, I wanted to have the section numbering in front of the equation, so that the equations are numbered as (Eq. 3.1) instead of (1).
I started from How to change equation numbering style.
\documentclass{article} \usepackage{amsmath} % \numberwithin{equation} doesn't exist without this package. \numberwithin{equation}{section} % This line resets equation numbering when starting a new section. \renewcommand{\theequation}{Eq. \thesection.\arabic{equation}} % This line ads "Eq." in front of your equation numbering. \begin{document} \section{Introduction} \begin{equation} 1+1=3? \end{equation} \begin{equation} 1+1\neq3 \end{equation} \section{Some section or chapter without content...} \section{Content!} \begin{equation} 1+1=2 \end{equation} \begin{equation} 1+2=3 \end{equation} \end{document} 
Unfortunately, the order of lines is crucial, because it is possible to override "\renewcommand{\theequation}{Eq. \thesection.\arabic{equation}}" with "\numberwithin{equation}{section}", and you only get (1.1).
Note that if "\numberwithin{equation}{section}" is left out, equation numbering continues after opening a new section, so it numbers like (Eq. 1.1), (Eq. 1.2), (Eq. 3.3), (Eq. 3.4), which is strange.
PS, I don't know why you would like to force a tag to become "equation 3.1", I thought the automatic numbering was a good thing.
THis worked for me Thank you
\documentclass{article} \usepackage{amsmath} \begin{document} \begin{equation}\tag{3.1} \begin{split} \log \mu_{ijk}= \lambda + \lambda_i ^I + \lambda_j ^J+\lambda_k ^K+\lambda_{ij} ^{IJ}+\lambda_{ik} ^{IK}+\lambda_{jk} ^{JK}+\lambda_{ijk} ^{IJK} \tag{3.1}\label{eq:3.1} \end{split} \end{equation} \end{document}
tag{\thesection.\theequation}; I think this has been asked before somewhere...