Btukfyl

I am using the book class to write a document that will have numbered paragraphs. Wanted the (auto-incremented) paragraph numbers to be bold and centred, then the text below, and the paragraph number to have a label so it can be referenced. Using other people's suggestions, I've got this function/command:

newcounter{para}

newcommandmypara[1]{

    parrefstepcounter{para}vfill

    begin{center}textbf{thepara}end{center}label{#1}

    }

and the way I use it in the document is:

mypara{myreference}

This is a nice numbered paragraph. I don't want any braces around the paragraph body, if at all possible.

which is working nicely EXCEPT when I use it close to the bottom of the page, in which case it's possible for the bold paragraph number to be on one page, and the text of the paragraph to be on the next. Is it possible to prevent a page break like this? I'm using multicols{2} as well, if that's relevant. Any help appreciated.

Min. example:

documentclass[10pt]{book}

usepackage{multicol}

setlengthcolumnsep{1cm}  % sets the distance between columns

usepackage{blindtext}



newcounter{para}

newcommandmypara[1]{

    parrefstepcounter{para}vfill

    begin{center}textbf{thepara}end{center}label{#1}

    }



title{Minimum working example}



begin{document}

    pagestyle{empty}

    % main part of the document:



    mainmatter

    begin{multicols}{2}



        mypara{ref1}

        blindtext



        mypara{ref2}

        blindtext



        mypara{ref3}

        blindtext



        mypara{ref4}

        blindtext



    end{multicols}

end{document}

Removing the vfill as pointed by @Flexo013 actually fixes your local problem.
I would however provide another solution, that IMO is more efficient.

First, if you don use chapter (and have no frontmatter) you definitely don't need the features of the book class. Second, if you wish to be able to produce a TOC and bookmarks, your para won't do the job.

Let me switch to article and modify the top-level sectioning command section by the use of a sinle line titleformat from titlesec package.
The MWE :

documentclass[10pt]{article}

usepackage[utf8]{inputenc}

usepackage{multicol}

setlengthcolumnsep{1cm}  % sets the distance between columns

usepackage{blindtext}

usepackage[explicit]{titlesec}

usepackage{hyperref}

pagestyle{empty}



begin{document}

begin{multicols}{2}



tableofcontents



makeatletter

titleformat{section}[block]{centeringbfseries}{thesection}{0pt}{@gobble{#1}}[label{par:#1}]

makeatother



    section{foo}

    blindtext



    section{bar}      

    In Sref{par:foo} we said that ldots



    section{foo2}      

     blindtext



    section{bar2}      

    In §,ref{par:foo2} we will say that ldots



    section{foo3}      

    blindtext



    section{bar3}      

     blindtext



    end{multicols}

end{document}

You can use the standard sectioning mechanism, suitably adapted.

documentclass[10pt]{book}

usepackage{multicol}

setlengthcolumnsep{1cm}  % sets the distance between columns

usepackage{blindtext}



makeatletter

newcounter{para}

newcommandmypara[1]{%

  @startsection{para}{1}{z@}%

    {-topsep}%

    {topsep}%

    {normalfontbfseriescentering}%

    {}label{#1}%

}

newcommand{myparamark}[1]{}

renewcommand{@seccntformat}[1]{%

  ifcsname format#1endcsname

    csname format#1endcsname

  else

    csname the#1endcsnamequad

  fi

}

newcommand{formatpara}{thepara}

makeatother





begin{document}



title{Minimum working example}

pagestyle{empty}



% main part of the document:



mainmatter



begin{multicols}{2}



mypara{ref1}

%A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more\

A line more



blindtext



mypara{ref2}

blindtext



mypara{ref3}

blindtext



mypara{ref4}

blindtext



end{multicols}



end{document}

You can check that uncommenting the first A line more, section 4 will be pushed to the next page.