# Displaying the binomial coefficient symbol in math mode

The problem is caused by the symbol of binomial coefficient (symbol of Newton), often used in math:

{N}\choose{k}


In my document I have formula:

$$P(A) = \sum P(\{ (e_1,...,e_N) \}) = {N}\choose{k} \cdot p^kq^{N-k}$$


which is rendered as: but should be: • Are you using LaTeX or plain TeX? – Torbjørn T. Aug 11 '13 at 14:14
• I am compiling pdfs from files by pdflatex. – MC2DX Aug 11 '13 at 14:20
• Is the symbol N really supposed to represent the physical unit for force (in which case you should render it in an upright rather than italic style), or are you using the symbol to denote an integer? – Mico Aug 11 '13 at 14:23
• I mean that I've cited the formula which is correct but pdflatex generate something which I don't want. I'm sorry, but I translated binomial coefficient directly from Polish. Sorry for misrepresentation. – MC2DX Aug 11 '13 at 14:28

To fix this, simply add a pair of braces around the whole binomial coefficient, i.e.

{N\choose k}


(The braces around N and k are not needed.)

However, as you're using LaTeX, it is better to use \binom from amsmath, i.e.

\binom{N}{k}


Further, it is not recommended to use $$...$$, see Why is $…$ preferable to $$? Last, I'll note that amsmath provides different commands for 'continuation dots', including \dotsc for triple dots between commas. \documentclass{article} \usepackage{amsmath} \begin{document} $P(A) = \sum P(\{ (e_1,\dotsc,e_N) \}) = \binom{N}{k} \cdot p^kq^{N-k}$ \end{document}  • What differs between \dots and \dotsc, with overleaf.com, the outputs are identical. – pzorba75 Mar 28 '18 at 3:08 • @pzorba75 The output of \dots depends on what it is placed between, try for example \documentclass{article} \usepackage{amsmath} \begin{document} 1,\dots,n 1+\dots+n \end{document}. Read also section 4.3 in the amsmath manual. – Torbjørn T. Mar 28 '18 at 6:28 Just add curly braces around it to separate it: $$ P(A) = \sum P(\{ (e_1,...,e_N) \})  =  {{N}\choose{k}} \cdot p^kq^{N-k}$$ That should fix it. • Hello @Lance Everhart! $$ ... $$ is obsolet. Use $...$  see l2tabu on page 6. – Su-47 Mar 27 '18 at 22:25 • Isn’t this exactly the same as the first line of the other answer? – GuM Mar 27 '18 at 22:25 • Hello @GuM! No. Before the braces { and } weren't escaped by the slash sign. – Su-47 Mar 27 '18 at 22:29 • @Su-47: I don’t want to seem obstinate, but the braces around (e_1,...,e_N) are correctly indicated as \{ and \} in the existing answer too… – GuM Mar 27 '18 at 23:18 • @Su-47 $$... is not obsolete, is TeX (and it should not be used in a LaTeX document, where you should use $...$). – CarLaTeX Mar 28 '18 at 3:10