Ttyjukl

In the book ``All of statistics A Concise Course in Statistical Inference'' of Larry Wasserman has a strange symbol, and look like a coil spring ... How can I use it in LaTeX?

Welcome to TeX-SE. I would like to draw your attention to the answers of this question. I base my proposal on the TikZ answer there. A simple (but scalable) coil can be produced with

documentclass{article}

usepackage{tikz}

usetikzlibrary{decorations.pathmorphing}

newcommandWasserman{tikz[baseline=-0.8ex]{draw[line width=0.07em,decorate,decoration={coil,segment

length=0.45em,amplitude=0.8ex}] (0,0) -- (2em,0);}}

begin{document}

[ X_1 Wasserman X_2]

end{document}

With some extra amount of work you can make it "boldable" and comply with the different math styles (the above version already becomes large if you say large, of course outside of math mode).

documentclass{article}

usepackage{tikz}

usetikzlibrary{decorations.pathmorphing}

usepackage{amsmath}

makeatletter

DeclareRobustCommand{checkbold}[1]{% https://tex.stackexchange.com/a/24635/121799

 edef@tempa{math@version}edef@tempb{bold}%

 ifx@tempa@tempb%

  def#1{1}%

 else

  def#1{0}%

 fi}

makeatother 

newcommand{WassermanCoil}{checkboldtmp%

ensuremath{mathrel{%

mathchoice{%

tikz[baseline=-0.8ex]{draw[line width={(1+0.33*tmp)*0.06em},decorate,decoration={coil,segment

length=0.45em,amplitude=0.8ex}] (0,0) -- (2em,0);}

}{%

tikz[baseline=-0.8ex]{draw[line width={(1+0.33*tmp)*0.06em},decorate,decoration={coil,segment

length=0.45em,amplitude=0.8ex}] (0,0) -- (2em,0);}

}{%

tikz[baseline=-0.6ex]{draw[line width={(1+0.33*tmp)*0.045em},decorate,decoration={coil,segment

length=0.36em,amplitude=0.6ex}] (0,0) -- (1.6em,0);}

}{%

tikz[baseline=-0.45ex]{draw[line width={(1+0.33*tmp)*0.035em},decorate,decoration={coil,segment

length=0.25em,amplitude=0.45ex}] (0,0) -- (1.2em,0);}

}}}}

begin{document}

[ X_1 WassermanCoil X_2 X_{Y_1 WassermanCoil Y_2}]

boldmath

[ X_1 WassermanCoil X_2 X_{Y_1 WassermanCoil Y_2}]

unboldmath



{Large AWassermanCoil B} {Huge AWassermanCoil B} {small AWassermanCoil B}



end{document}

One could go even more fancy by employing a 3d coil but I am not going to spell this out unless asked to.

Another thing you can do is to follow CarLaTeX's answer and use trimpclip to clip away one coil of the gluon (that has been mentioned in the comments just now) to get

documentclass{article}

usepackage{amsmath}

usepackage[nointegrals]{wasysym}    

usepackage{trimclip}

newcommand{WassermanCoil}{clipbox{0em -1ex 1.65em -1ex}{gluon}%

clipbox{1.7em -1ex 0em -1ex}{gluon}}

begin{document}

[

X_1WassermanCoil X_2

]

end{document}

If the symbol gluon hadn't existed, or you drew the symbol with TikZ or you can use, for example, the package circuitikz. Adding also the scalerel package allows you to adjust the size of the symbol as you wish.

documentclass{article} 

usepackage{circuitikz}

usepackage{scalerel}

newcommand{ngluon}{scaleobj{.7}{begin{tikzpicture} 

    draw (.65,0) to [cute inductor] (1.5,0) ; 

end{tikzpicture}}}

begin{document} 

$X_1,ngluon, X_2|X_3$

end{document}