Is there a command to write the form of a combination or permutation?

You could use the \prescript command from the mathtools package and define two commands; something along the following lines:

\documentclass{article}
\usepackage{mathtools}

\newcommand\Myperm[2][^n]{\prescript{#1\mkern-2.5mu}{}P_{#2}}
\newcommand\Mycomb[2][^n]{\prescript{#1\mkern-0.5mu}{}C_{#2}}

\begin{document}

\[
\Myperm{k} = \frac{n!}{(n-k)!}\quad
\Mycomb{k} = \frac{n!}{k!(n-k)!}\quad
\Myperm[m]{k} = \frac{m!}{(m-k)!}\quad
\Mycomb[m]{k} = \frac{m!}{k!(m-k)!}\quad
\]

\end{document}

You can define your own:

Code:

\documentclass{article}
\usepackage{amsmath}

\newcommand*{\Perm}[2]{{}^{#1}\!P_{#2}}%
\newcommand*{\Comb}[2]{{}^{#1}C_{#2}}%

\begin{document}  
$\Perm{n}{k}=\frac{n!}{(n-k)!}$ - permutation 

$\binom nk=\Comb{n}{k}=\frac{n!}{k!(n-k)!}$ - combination  
\end{document}

I provide a generic \permcomb macro that will be used to setup \perm and \comb.

The spacing is between the prescript and the following character is kerned with the help of \mkern.

The default kerning between the prescript and P is -3mu, and -1mu with C, which can be changed by using the optional argument of all three macros.

Code

\documentclass{article}
\usepackage{amsmath}
\newcommand*{\permcomb}[4][0mu]{{{}^{#3}\mkern#1#2_{#4}}}
\newcommand*{\perm}[1][-3mu]{\permcomb[#1]{P}}
\newcommand*{\comb}[1][-1mu]{\permcomb[#1]{C}}
\begin{document}
$\perm{n}{k}$

$\comb{n}{k}$

$\permcomb[-3mu]{J}{l}{k}$
\end{document}

Output