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My minimum working example:

\documentclass{report}

\usepackage{amsmath}

\begin{document}

\begin{center}
\boxed{K_{k+1}=P_ka_{k+1}^T(a_{k+1}P_ka_{k+1}^T+I_N)^{-1}}
\end{center}

\end{document}

The result:

enter image description here

Now how can I number this equation?

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3 Answers 3

up vote 16 down vote accepted

Just replace center environment with equation. I think it looks a little better with larger parentheses.

Edit: It might be easier to read as a fraction - except that it seems to be an equation about matrices ...

\documentclass{report}

\usepackage{amsmath}

\begin{document}

\begin{equation}
\boxed{K_{k+1}=P_ka_{k+1}^T\left(a_{k+1}P_ka_{k+1}^T+I_N\right)^{-1}}
\end{equation}

\begin{equation}
\boxed{K_{k+1}=\frac{P_ka_{k+1}^T}{a_{k+1}P_ka_{k+1}^T+I_N}}
\end{equation}

\end{document}

enter image description here

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If you want a costumized box, you can use the empheq package by Morten Høgholm:

\documentclass{article}

\usepackage{mathtools}
\usepackage[
  amsmath
]{empheq}
\usepackage{xcolor}

\definecolor{shadecolor}{cmyk}{0,0,0.45,0}
\definecolor{light-blue}{cmyk}{0.25,0,0,0}
\newsavebox{\mysaveboxM}
\newsavebox{\mysaveboxT}
\newcommand*\Garybox[2][A Nice Box]{%
  \sbox{\mysaveboxM}{#2}%
  \sbox{\mysaveboxT}{\fcolorbox{black}{light-blue}{#1}}%
  \sbox{\mysaveboxM}{%
    \parbox[b][\ht\mysaveboxM+0.5\ht\mysaveboxT+0.5\dp\mysaveboxT][b]{%
      \wd\mysaveboxM}{#2}%
  }%
  \sbox{\mysaveboxM}{%
    \fcolorbox{black}{shadecolor}{%
      \makebox[\linewidth-17.5em]{\usebox{\mysaveboxM}}%
    }%
  }%
  \usebox{\mysaveboxM}%
  \makebox[0pt][r]{%
    \makebox[\wd\mysaveboxM][c]{%
      \raisebox{\ht\mysaveboxM-0.5\ht\mysaveboxT
                +0.5\dp\mysaveboxT-0.5\fboxrule}{\usebox{\mysaveboxT}}%
    }%
  }%
}

\begin{document}

\begin{empheq}[box = {\Garybox[A Nice Box]}]{equation}
  K_{k+1}
  = P_{k}a_{k+1}^{T}\left(a_{k+1}P_{k}a_{k+1}^{T}+I_{N}\right)^{-1}
\end{empheq}

\end{document}

output

Note that the example can be found on pages 23--24 of the package manual.

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With tcolorbox help it's possible to include equation numbers in framed environments. It provides options ams equation, ams gather, ams align and corresponding starred versions. It's also possible to combine tcolorbox with empheq package.

\documentclass{report}

\usepackage{amsmath}
\usepackage[most]{tcolorbox}

\tcbset{colback=yellow!10!white, colframe=red!50!black, 
        highlight math style= {enhanced, %<-- needed for the ’remember’ options
            colframe=red,colback=red!10!white,boxsep=0pt}
        }

\begin{document}

\begin{equation}
\tcboxmath{K_{k+1}=P_ka_{k+1}^T(a_{k+1}P_ka_{k+1}^T+I_N)^{-1}}
\end{equation}

\begin{tcolorbox}[ams equation]
K_{k+1}=P_ka_{k+1}^T(a_{k+1}P_ka_{k+1}^T+I_N)^{-1}
\end{tcolorbox}

\begin{tcolorbox}[ams align]
\sum\limits_{n=1}^{\infty} \frac{1}{n} &= \infty.\\
\int x^2 ~\text{d}x &= \frac13 x^3 + c.
\end{tcolorbox}

\begin{tcolorbox}[ams gather]
\sum\limits_{n=1}^{\infty} \frac{1}{n} = \infty.\\
\int x^2 ~\text{d}x = \frac13 x^3 + c.
\end{tcolorbox}

\begin{align*}
\tcbhighmath[remember as=fx]{f(x)}
&= \int\limits_{1}^{x} \frac{1}{t^2}~dt
= \left[ -\frac{1}{t} \right]_{1}^{x}\\
&= -\frac{1}{x} + \frac{1}{1}\\
&=
\tcbhighmath[remember,overlay={%
\draw[blue,very thick,->] (fx.south) to[bend right] ([yshift=2mm]frame.west);}]
{1-\frac{1}{x}.}
\end{align*}

\end{document}

enter image description here

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