Gr.postscript("28505-cartesio.ps",4,4)
lato=c(-2,2); par(bg="yellow")
Gr(lato,lato,cornice=T)
t=seq(-2,2,by=0.01)
f = function (x,y) x^3+y^3-3*x*y
valori=outer(t,t,f)
Gr.livelli(t,t,valori=valori,
  scritte=0.5,
  livelli=setdiff(seq(-2,2,by=0.5),0))
Gr.livelli(t,t,valori=valori,
  spessore=2,colore="red",scritte=0)
dev.off()
---------------------------------------
phi = function (t)
3*t*(1+1i*t)/(1+t^3)

Gr.postscript("28505-cartesio-par.ps",
  4,4)
lato=c(-3,3); par(bg="yellow")
Gr(lato,lato,cornice=T)
x1=Re(phi(15)); y1=Im(phi(15))
x2=Re(phi(20)); y2=Im(phi(20))
arrows(x1,y1,x2,y2,
  length=0.08,angle=20,col='red')
x1=Re(phi(-1.7)); y1=Im(phi(-1.7))
x2=Re(phi(-1.6)); y2=Im(phi(-1.6))
arrows(x1,y1,x2,y2,
  length=0.08,angle=20)
t=seq(-0.7,50,by=0.01)
lines(phi(t),col='red')
t=seq(-20,-1.6,by=0.01)
text(-1.3+1.3i,
  expression(group('(',
  list(-1,infinity),')')),cex=0.6)
text(1.2-1i,
  expression(group('(',
  list(-infinity,-1),')')),cex=0.6)
lines(phi(t))
dev.off()