Symbol inside math operator

Question

I would like to define a math operator that looks a bit like this, which I mocked up in an image editor from the amsmath square symbol:

Sometimes there will be a different letter than lambda inside the box, or an expression like lambda', but the box should always be the same size, and always be square - it's meant to be an inline operator with a symbol drawn on it, not a box around part of a formula.

I don't mind if I have to manually position the formula inside it, as there will probably only be a few different letters that I'll use.

It doesn't have to be based on square. In fact I think it would look nicer if the baseline was a bit lower, to match + for example.

Is there a reasonable way to achieve this?

Vincent · Accepted Answer

Here's a solution using a framebox of fixed size. I thought it would look better if the symbol in the square was smaller, in scriptstyle. I think it looks reasonably good with any lowercase latin or greek letter.

documentclass{article}
usepackage{amsmath}
newcommand{squareop}[1]{%
    setlength{fboxsep}{0pt}%
    setlength{unitlength}{.7em}%
    mathrel{%
        raisebox{-1pt}{framebox(1,1){(scriptstyle #1)}}%
    }%
}
begin{document}
begin{tabular}{lll}
    ( x squareop{a} y )  & ( x squareop{b} y )    & ( x squareop{c} y )    
    ( x squareop{d} y )  & ( x squareop{e} y )    & ( x squareop{f} y )    
    ( x squareop{g} y )  & ( x squareop{h} y )    & ( x squareop{i} y )    
    ( x squareop{j} y )  & ( x squareop{k} y )    & ( x squareop{l} y )    
    ( x squareop{m} y )  & ( x squareop{n} y )    & ( x squareop{o} y )    
    ( x squareop{p} y )  & ( x squareop{q} y )    & ( x squareop{r} y )    
    ( x squareop{s} y )  & ( x squareop{t} y )    & ( x squareop{u} y )    
    ( x squareop{v} y )  & ( x squareop{w} y )    & ( x squareop{z} y )    
end{tabular}
qquad
begin{tabular}{lll}
    ( x squareop{alpha} y )     & ( x squareop{beta} y )    & ( x squareop{gamma} y )  
    ( x squareop{delta} y )     & ( x squareop{epsilon} y ) & ( x squareop{zeta} y )   
    ( x squareop{eta} y )       & ( x squareop{theta} y )   & ( x squareop{iota} y )   
    ( x squareop{kappa} y )     & ( x squareop{lambda} y )  & ( x squareop{mu} y )     
    ( x squareop{nu} y )        & ( x squareop{xi} y )      & ( x squareop{pi} y )     
    ( x squareop{rho} y )       & ( x squareop{sigma} y )   & ( x squareop{tau} y )    
    ( x squareop{upsilon} y )   & ( x squareop{phi} y )     & ( x squareop{varphi} y ) 
    ( x squareop{chi} y )       & ( x squareop{psi} y )     & ( x squareop{omega} y )  
end{tabular}
end{document}

egreg · Answer

You may also want the big version.

documentclass{article}
usepackage{amsmath,array,relsize}

makeatletter
DeclareRobustCommand{boxop}[1]{mathbin{mathpalettebox@op{#1}}}
DeclareRobustCommand{bigboxop}[1]{mathop{mathpalettebigbox@op{#1}}slimits@}

newcommand{box@op}[2]{%
  begingroup
  sboxz@{$m@th#1mkern15mu$}%
  dimen@=wdz@
  setlength{fboxsep}{0pt}%
  makebox[dimen@]{%
    framebox[0.9dimen@]{%
      vbox to 0.9dimen@{%
        vss
        hbox{raisebox{depth}{$box@op@style{#1}#2$}}%
        vss
      }%
    }%
  }%
  endgroup
}
newcommand{box@op@style}[1]{%
  ifx#1displaystylescriptstyleelse
  ifx#1textstylescriptstyleelse
  scriptscriptstylefifim@th
}
newcommand{bigbox@op}[2]{%
  begingroup
  sboxz@{$m@th#1sum$}%
  dimen@=wdz@
  vphantom{sum}%
  vcenter{%
    setlength{fboxsep}{0pt}%
    hbox to dimen@{%
      hss
      framebox[0.9dimen@]{%
        vbox to 0.9dimen@{%
          vss
          hbox{raisebox{depth}{$m@th#1box@op@larger{#1}{#2}$}}%
          vss
        }%
      }%
      hss
    }%
  }%
  endgroup
}
newcommand{box@op@larger}[2]{%
  ifx#1displaystyle
    expandafter@firstoftwo
  else
    expandafter@secondoftwo
  fi
  {mathlarger{#2}}{#2}%
}
makeatother

begin{document}

$xboxop{lambda}yboxop{lambda!'}zboxop{varphi}wboxop{beta}u$

$scriptstyle xboxop{lambda}yboxop{lambda!'}z$

$displaystylesum_{k=1}^nbigboxop{lambda}_{k=1}^n x_k$
$bigboxop{lambda}_{k=1}^n x_k$
$scriptstylebigboxop{lambda}_{k=1}^n x_k$

$displaystylesum_{k=1}^nsum_{k=1}^n x_k$

end{document}

Sebastiano · Answer

Using youngtab that I not knew with the symbol inside math operator. It is right to write that the square has the same dimension.
documentclass[a4paper,12pt]{article}
usepackage{amsmath,amssymb}

usepackage{youngtab}
Ylinethick{0.5pt}
begin{document}

$a mathrel{young(lambda)} b$, $A_{mathrel{young(mu)_c^d}}$, $Xsimsum_{i=1}^nx_imathrel{young(diamond)}y_i$.

end{document}

Symbol inside math operator

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