code for figure

> p1:=plot(x^3-x,x=-1.5..1.5,thickness=3,color=black):
p1a:=plot({-4,4},x=-2..2,thickness=3,color=black):
p2:=plot(.4,x=-1..0,thickness=3):
p3:=plot(-.4,x=0..1,thickness=3):
lns:=plot({[[-1.5,4],[-1.5,-4]],[[ 1.5,4],[1.5,-4]]},style=line,thickness=3, linestyle=3,color=black):
lnsa:=plot({[[-2,4],[-2,-4]],[[ 2,4],[2,-4]]},style=line,thickness=3, linestyle=0,color=black):
txt:=plots[textplot](
{[-1.5,-4.3,`x=a`],[ 1.5,-4.3,`x=b`]},font=[TIMES,BOLD,16]):
plots[display]({p1,p1a,p2,p3,lns,lnsa,txt},axes=none);