Gr.postscript("28505-y2=x3-x2or.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) y^2-x^3+x^2
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)
points(0,0,pch=20,cex=0.8,col='red')
dev.off()