-10 12 setxrange
-10 10 setyrange

quadrillage
/unites {} def
3 3 setmkstep
tracerepere
marks

/f {
   dup 1 add 2 exch div add 2 sub 
}  def
/f' {
   setxvar
   x dup mul 2 x mul add -1 add
   x 1 add dup mul div
} def

/x1 -1 2 sqrt sub def
/x2 -1 2 sqrt add def

/dotscale {.5 dup} def
4 settailletangente
x1 (f) tangente
x2 (f) tangente
8 settailletangente
1 (f) tangente

gsave
   pointilles
   [x1 dup f x1 0] ligne
grestore

1 setlinewidth
xmin -1.0001 {f} Courbe
-.999 xmax {f} Courbe

setTimesItalic
   (O) O drtext
   (C) 10 6 cctext
   (f) indice
setSymbol
   (D) -2.5 -1.5 cltext