# What is the best way to write this equation to have a better look?

What is the best way to write the following equation:

\begin{flalign}
\begin{aligned}
&\sum_{i=1}^{N-N^*} \Bigg(\bigg[\sum_{j=2}^{d_i-1} -\;\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{id_i}'(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})}{(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})+\bm{P}_{id_i}'\bm{P}_{id_i}}
\mathbf{\tilde{Y}}_{d_i} \mathbf{\tilde{Y}}'_{d_i}\bigg]\\
&-\bigg[\sum_{j=2}^{d_i-1} \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j+\tfrac{(1-\bm{P}_{id_i})}{\bm{P}'_{id_i}}
\mathbf{\tilde{Y}}_{d_i} \bm{P}_{id_i}(1-\bm{P}_{id_i}) \mathbf{\tilde{Y}}'_{d_i} \bigg]\Bigg)\\
&-\sum_{i=1}^{N^*} \sum_{j=2}^{n_i}\bigg[\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j\bigg]
\end{aligned}
&&
\end{flalign}


Following is an image to how it looks like in the way I wrote the code above

I tried also to use multiline but did not get a nice look either.

I change the code using footnotesize, the new code is

{\footnotesize
\begin{flalign}  \label{18}
\begin{aligned}
\frac{\partial^2 \ell}{\partial \bm{\phi}\partial\bm{\phi}'}&=\sum_{i=1}^{N-N^*} \Bigg(\bigg[\sum_{j=2}^{d_i-1} -\;\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{id_i}'(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})}{(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})+\bm{P}_{id_i}'\bm{P}_{id_i}}
\mathbf{\tilde{Y}}_{d_i} \mathbf{\tilde{Y}}'_{d_i}\bigg]\\
\mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j+\tfrac{(1-\bm{P}_{id_i})}{\bm{P}'_{id_i}}
\mathbf{\tilde{Y}}_{d_i} \bm{P}_{id_i}(1-\bm{P}_{id_i}) \mathbf{\tilde{Y}}'_{d_i} \bigg]\Bigg)\\
&\qquad-\sum_{i=1}^{N^*} \sum_{j=2}^{n_i}\bigg[\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j\bigg]
\end{aligned}
&&
\end{flalign}
}


It looks now as:

• Please do not post code snippets - create minimal working examples (MWEs) instead. – epR8GaYuh Dec 18 '18 at 13:29
• I post it as it is because the equation is splitted in many line. I can't compile the above code here to show how it is very messy – N. I. ElZayat Dec 18 '18 at 13:33
• Please state, in words, what you would like to achieve. Please also state what the main font size is and how wide the text block is. – Mico Dec 18 '18 at 13:37
• Do you use the default layout (with a very large outer margin) or do you customise your layout (say with geometry)? – Bernard Dec 18 '18 at 13:54
• The combination of \footnotesize and \tfrac makes reading the material quite a chore -- unless a free magnifying glass is handed out... – Mico Dec 18 '18 at 15:06

Do six lines. I changed the position of some of the brackets from before to after the summation sign and reduced the size.

Note that it should be \bigl for a left delimiter and \bigr for a right one (with the obvious variations for Big, bigg and Bigg).

\documentclass{article}
\usepackage{amsmath,bm}

\begin{document}

\begin{flalign}
\begin{aligned}
\frac{\partial^2 \ell}{\partial \bm{\phi}\,\partial\bm{\phi}'}={}&
\begin{aligned}[t]
\sum_{i=1}^{N-N^*}
\biggl(\,\sum_{j=2}^{d_i-1}\Bigl[
& -\frac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}
{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}
\tilde{\mathbf{Y}}_j \tilde{\mathbf{Y}}'_j
\\
& +\frac{\bm{P}_{id_i}'(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})}
{(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})+\bm{P}_{id_i}'\bm{P}_{id_i}}
\tilde{\mathbf{Y}}_{d_i} \tilde{\mathbf{Y}}'_{d_i}\Bigr]
\\
-\sum_{j=2}^{d_i-1}\Bigl[
& \frac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\tilde{\mathbf{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \tilde{\mathbf{Y}}'_j
\\
&  +\frac{(1-\bm{P}_{id_i})}{\bm{P}'_{id_i}}
\tilde{\mathbf{Y}}_{d_i} \bm{P}_{id_i}(1-\bm{P}_{id_i}) \tilde{\mathbf{Y}}'_{d_i} \Bigr]\biggr)
\end{aligned}
\\
&\begin{aligned}[t]
-\sum_{i=1}^{N^*} \sum_{j=2}^{n_i}\Bigl[
& \frac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}
{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}
\tilde{\mathbf{Y}}_j \tilde{\mathbf{Y}}'_j
\\
& + \frac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\tilde{\mathbf{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \tilde{\mathbf{Y}}'_j\Bigr]
\end{aligned}
\end{aligned}
&&
\end{flalign}

\end{document}


I changed all \mathbf{\tilde{Y}} to \tilde{\mathbf{Y}} that's conceptually sounder. You may also want to use \widetilde, but I don't recommend so.

• I'd be tempted to replace all instances of \mathbf{\tilde{Y}} with \widetilde{\mathbf{Y}}. – Mico Dec 18 '18 at 14:14
• @Mico It should be \tilde{\mathbf{Y}}, much likely. – egreg Dec 18 '18 at 14:16
• Nice touch to supply the missing inner brackets, by the way. – Mico Dec 18 '18 at 14:18
• @egreg it is really looks very nice – N. I. ElZayat Dec 18 '18 at 14:32

Reduce the text size or make it spread into several lines.

\begin{flalign}
{\footnotesize
\begin{aligned}
\frac{\partial^2 \ell}{\partial \bm{\phi}\partial\bm{\phi}'}=
&\sum_{i=1}^{N-N^*} \Bigg(\bigg[\sum_{j=2}^{d_i-1} -\;\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{id_i}'(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})}{(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})+\bm{P}_{id_i}'\bm{P}_{id_i}}\\&
\mathbf{\tilde{Y}}_{d_i} \mathbf{\tilde{Y}}'_{d_i}\bigg]
-\bigg[\sum_{j=2}^{d_i-1} \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j+\tfrac{(1-\bm{P}_{id_i})}{\bm{P}'_{id_i}}
\mathbf{\tilde{Y}}_{d_i} \bm{P}_{id_i}(1-\bm{P}_{id_i}) \mathbf{\tilde{Y}}'_{d_i} \bigg]\Bigg)\\
&-\sum_{i=1}^{N^*} \sum_{j=2}^{n_i}\bigg[\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'}
\mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j\bigg]
\end{aligned}}
\end{flalign}

• I just notice your answer, this is exactly what comes to my mind before I saw you answer – N. I. ElZayat Dec 18 '18 at 14:06
• And you remembered to check the log before you posted this? Hint: \footnotesize is not allowed in math mode. – daleif Dec 18 '18 at 14:08
• No error is not the same as not being correct. You get LaTeX Font Warning: Command \footnotesize invalid in math mode if you look in the log. Never use text font resizing commands in math mode. – daleif Dec 18 '18 at 14:16
• It is still a bad practise that should be avoided in a good document design. – daleif Dec 18 '18 at 14:47
• Shrinking the font to make something fit is a bad idea regardless. – JPi Dec 18 '18 at 17:04

I've made the code a bit more legible and changed \Bigg( to \Biggl( etc. Also, I'm using the multline environment from amsmath which is suitable here.

\documentclass{article}

\usepackage{amsmath}
\usepackage{bm}
\usepackage[margin=1in]{geometry}

\newcommand{\bP}{\bm{P}}
\newcommand{\tY}{\mathbf{\tilde Y}}
\begin{document}
\begin{multline}
\frac{\partial^2 \ell}{\partial \bm{\phi}\partial\bm{\phi}'}=
\sum_{i=1}^{N-N^*} \Biggl(\biggl[\sum_{j=2}^{d_i-1} -\;\frac{\bP_{ij}'\bP_{ij} (1-\bP_{ij})'}{(1-\bP_{ij})'(1-\bP_{ij})+\bP_{ij}'\bP_{ij}}\tY_j \tY'_j + \frac{\bP_{id_i}'(1-\bP_{id_i})'(1-\bP_{id_i})}{(1-\bP_{id_i})'(1-\bP_{id_i})+\bP_{id_i}'\bP_{id_i}}
\tY_{d_i} \tY'_{d_i}\biggr]\\
-\biggl[\sum_{j=2}^{d_i-1} \frac{\bP_{ij}}{(1-\bP_{ij})'}
\tY_j \bP_{ij}(1-\bP_{ij}) \tY'_j+\frac{(1-\bP_{id_i})}{\bP'_{id_i}}
\tY_{d_i} \bP_{id_i}(1-\bP_{id_i}) \tY'_{d_i} \biggr]\Biggr)\\
-\sum_{i=1}^{N^*} \sum_{j=2}^{n_i}\biggl[\frac{\bP_{ij}'\bP_{ij} (1-\bP_{ij})'}{(1-\bP_{ij})'(1-\bP_{ij})+\bP_{ij}'\bP_{ij}}\tY_j \tY'_j + \frac{\bP_{ij}}{(1-\bP_{ij})'}
\tY_j \bP_{ij}(1-\bP_{ij}) \tY'_j\biggr]
\end{multline}

\end{document}


I propose this, with package geometry. If you have the default layout with large margins, you can nest the equation contents in a medsize environment from nccmath (~ 80% of previous size):

\documentclass{article}
\usepackage{geometry}
\usepackage{showframe}
\renewcommand{\ShowFrameLinethickness}{0.3pt}
\usepackage{amsmath, bm, nccmath}
\usepackage{diffcoeff}

\begin{document}

\begin{aligned}\diffp[1,1]{\ell}{\bm{\phi},\bm{\phi}'} = \sum_{i=1}^{N-N^*} \Bigg( & \bigg[\begin{aligned}[t]\sum_{j=2}^{d_i-1} -\;\tfrac{\bm{P}_{ij}' \bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{id_i}'(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})}{(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})+\bm{P}_{id_i}'\bm{P}_{id_i}} \mathbf{\tilde{Y}}_{d_i} \mathbf{\tilde{Y}}'_{d_i} & \bigg]\\ %& - \bigg[\sum_{j=2}^{d_i-1} \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'} \mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j+\tfrac{(1-\bm{P}_{id_i})}{\bm{P}'_{id_i}} \mathbf{\tilde{Y}}_{d_i} \bm{P}_{id_i}(1-\bm{P}_{id_i}) \mathbf{\tilde{Y}}'_{d_i} &\bigg]\Bigg) \end{aligned}\\ % & -\sum_{i=1}^{N^*} \sum_{j=2}^{n_i} & \bigg[\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'} \mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j\bigg] \end{aligned}
\bigskip

\begin{medsize} \begin{aligned}\diffp[1,1]{\ell}{\bm{\phi},\bm{\phi}'} = \sum_{i=1}^{N-N^*} \Biggl( & \Biggl[\begin{aligned}[t]\sum_{j=2}^{d_i-1} -\;\tfrac{\bm{P}_{ij}' \bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{id_i}'(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})}{(1-\bm{P}_{id_i})'(1-\bm{P}_{id_i})+\bm{P}_{id_i}'\bm{P}_{id_i}} \mathbf{\tilde{Y}}_{d_i} \mathbf{\tilde{Y}}'_{d_i} & \Biggr]\\ %& - \Biggl[\sum_{j=2}^{d_i-1} \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'} \mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j+\tfrac{(1-\bm{P}_{id_i})}{\bm{P}'_{id_i}} \mathbf{\tilde{Y}}_{d_i} \bm{P}_{id_i}(1-\bm{P}_{id_i}) \mathbf{\tilde{Y}}'_{d_i} &\Biggr]\Biggr) \end{aligned}\\ % & -\sum_{i=1}^{N^*} \sum_{j=2}^{n_i} & \biggl[\tfrac{\bm{P}_{ij}'\bm{P}_{ij} (1-\bm{P}_{ij})'}{(1-\bm{P}_{ij})'(1-\bm{P}_{ij})+\bm{P}_{ij}'\bm{P}_{ij}}\mathbf{\tilde{Y}}_j \mathbf{\tilde{Y}}'_j + \tfrac{\bm{P}_{ij}}{(1-\bm{P}_{ij})'} \mathbf{\tilde{Y}}_j \bm{P}_{ij}(1-\bm{P}_{ij}) \mathbf{\tilde{Y}}'_j\biggr] \end{aligned} \end{medsize}

\end{document}