Why the font in my equation is different?

Question

My code is

\newcommand\rsx[1]{\left.{#1}\vphantom{\Big|}\right|}
\begin{eqnarray}
&&let \;\; L(\bf{X},\lambda,\mu)= f(\bf{X})+\sum_{i=1}^p \lambda_{i}h_{i}(\bf{X})+\sum_{j=1}^q\mu_{j}g_{j}(\bf{X}) \nonumber\\
&& \;\;\;\;\; \rsx{\frac{\partial L}{\partial \bf{X}}}_{\bf{X}=\bf{X^*}}=0 \nonumber\\
&& \;\;\;\;\; \lambda_{i}\neq0,\;\;\;\mu_{j}\geq0,\;\;\;\nonumber\\
&& \;\;\;\;\; \mu_{j}g_{j}(\bf{X^*})=0,\;\;\; h_{i}(\bf{X}^*)=0 \;\;\; g_{j}(\bf{X^*})\leq0
\label{equ: KKT conditions}
\end{eqnarray}

and below is the result, as can be seen in this figure, the font type is different in these equations, why (see the letter "g" in different lines)?

Answer 1

The alignment points could get some nicer fine-tuning, but I just wanted to correct some issues in your code. This is how I would type your formula:

% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}
\newcommand{\pder}[2][]{\frac{\partial#1}{\partial#2}}
\newcommand\numberthis{\addtocounter{equation}{1}\tag{\theequation}}

\begin{document}
\begin{align*}
\MoveEqLeft[7]\text{let } L(\mathbf{X},\lambda,\mu)= f(\mathbf{X})+\sum_{i=1}^p \lambda_{i}h_{i}(\mathbf{X})+\sum_{j=1}^q\mu_{j}g_{j}(\mathbf{X})\\
    \pder[L]{\mathbf{X}}\biggr|_{\mathbf{X}=\mathbf{X}^*}&=0\\
    \lambda_{i}&\neq 0,\quad\mu_{j}\geq 0,\\
    \mu_{j}g_{j}(\mathbf{X}^*)&=0,\quad h_{i}(\mathbf{X}^*)=0,\quad g_{j}(\mathbf{X}^*)\leq 0 \numberthis\label{equ:KKTconditions}
\end{align*}
\end{document}

---

or:

\begin{align*}
\MoveEqLeft[2]\text{let } L(\mathbf{X},\lambda,\mu)= f(\mathbf{X})+\sum_{i=1}^p \lambda_{i}h_{i}(\mathbf{X})+\sum_{j=1}^q\mu_{j}g_{j}(\mathbf{X})\\
    &\pder[L]{\mathbf{X}}\biggr|_{\mathrlap{\mathbf{X}=\mathbf{X}^*}}=0\\
    &\lambda_{i}\neq 0,\quad\mu_{j}\geq 0,\\
    &\mu_{j}g_{j}(\mathbf{X}^*)=0,\quad h_{i}(\mathbf{X}^*)=0,\quad g_{j}(\mathbf{X}^*)\leq 0 \numberthis\label{equ:KKTconditions}
\end{align*}

Answer 2

The command \bf is a Plain-TeX command. While it continues to work in LaTeX, it's considered deprecated for use in LaTeX.

The immediate cause of the problem you've encountered is an incorrect use of \bf: it is a switch and doesn't take an argument. Hence, you should write {\bf ...} instead of \bf{...}. As you've "discovered", the scope of \bf{...} ... is not the material inside the curly braces; it extends to the end of the current (math) group.

For a LaTeX document, one shouldn't use \bf anymore. Write \textbf{...} or {\bfseries ...} for text material. For math material, use \mathbf{...} if you want upright bold letters and \bm{...} (which requires the bm package) if you want italic bold letters.

There are two additional issues with your code snippet:

• Don't use the eqnarray environment: it's badly deprecated. Use an environment such as align instead. See the posting eqnarray vs align for a thorough examination of the reasons for not using eqnarray.

• You seem to be engaging in quite a bit of visual formatting. This makes the code both hard to read and less flexible than you would probably like it to be. The code below suggests some alternative measures, and uses align instead of eqnarray. It also shows the different effects generated by \mathbf and \bm.

\documentclass{article}
\usepackage{amsmath}  % for 'align' environment
\usepackage{bm}
\newcommand\rsx[1]{#1\vphantom{\Big|}\bigg\rvert}

\begin{document}

First with \verb+\bm{...}+:
\begin{align}
\text{Let }&L(\bm{X},\lambda,\mu)= f(\bm{X})
  +\sum_{i=1}^p \lambda_{i}h_{i}(\bm{X})
  +\sum_{j=1}^q\mu_{j}g_{j}(\bm{X}) \nonumber\\
&\rsx{\frac{\partial L}{\partial\bm{X}}}
  _{\bm{X}=\bm{X^*}} =0 \nonumber\\
&\lambda_{i}\neq0,\quad \mu_{j}\geq0,\nonumber\\
&\mu_{j} g_{j}(\bm{X^*})=0,\quad 
  h_{i}(\bm{X}^*)=0,\quad 
  g_{j}(\bm{X^*})\leq0
\label{equ:KKT_conditions}
\end{align}

\bigskip
Second with \verb+\mathbf{...}+
\begin{align}
\text{Let }&L(\mathbf{X},\lambda,\mu)= f(\mathbf{X})
  +\sum_{i=1}^p \lambda_{i}h_{i}(\mathbf{X})
  +\sum_{j=1}^q\mu_{j}g_{j}(\mathbf{X}) \nonumber\\
&\rsx{\frac{\partial L}{\partial\mathbf{X}}}
  _{\mathbf{X}=\mathbf{X^*}} =0 \nonumber\\
&\lambda_{i}\neq0,\quad \mu_{j}\geq0,\nonumber\\
&\mu_{j} g_{j}(\mathbf{X^*})=0,\quad 
  h_{i}(\mathbf{X}^*)=0,\quad 
  g_{j}(\mathbf{X^*})\leq0
\label{equ:KKT_conditions_alt}
\end{align}

\end{document}