How to write a subscript/superscript before the main symbol?

Question

In LaTeX, Is there a way to write a subscript (or superscript) before (i.e. to the left of) the main symbol as in the n_C_k (n choose k) below?

Answer 1

You could use

{n \choose k} = C^n_k = {}_nC_k

to typeset your example.

Answer 2

You can use \sideset as far the main symbol is a math operator:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[\sideset{_1^2}{_3^4}\sum\]
\[\sideset{_a^b}{_c^ d}{\mathop{\kern-.35em\text{\huge C}}}\]
\end{document}

Answer 3

I add my suggestion to have the same output of your image using leftidx package. You can see that the alignment of the subscript is the same.

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,leftidx}

\begin{document}
\[\dbinom{n}{k}=C^{n}_{k}=\leftidx{_n}{C}{_k}\]
\[\tbinom{n}{k}=C^{n}_{k}=\leftidx{_n}{C}{_k}\]
\[{n \choose k}=C^{n}_{k}=\leftidx{_n}{C}{_k}\]
\end{document}