Take the 2-minute tour ×
TeX - LaTeX Stack Exchange is a question and answer site for users of TeX, LaTeX, ConTeXt, and related typesetting systems. It's 100% free, no registration required.

I'm trying to write

^nP_k=\frac{n!}{(n-k)!}  
\binom nk=^nC_k=\frac{n!}{k!(n-k)!}

but when compiled the n is a little far away from the P and C for my liking. Is there a command to write this? I know there is a \binom so I was hopeful. If not, is there a way to force the n to be closer?

\documentclass{article}  
\begin{document}  
$ ^nP_k=\frac{n!}{(n-k}!} - permutation \\  
\binom nk=^nC_k=\frac{n!}{k!(n-k)!} - combination $  
end{document}
share|improve this question
add comment

3 Answers 3

up vote 5 down vote accepted

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}

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

\end{document}

enter image description here

share|improve this answer
    
It's not on topic, I've just noticed \qquad a few times and wondered what it is? –  DannyBland Apr 6 '13 at 0:55
    
@DannyBland \qquad introduces (both in text and in math mode) a space equal to twice \quad. –  Gonzalo Medina Apr 6 '13 at 0:57
add comment

You can define your own:

enter image description here

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}
share|improve this answer
add comment

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

enter image description here

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.