###############  EN S-PLUS ###############
function.draw<-
function (f, low=-1, hi=1, n=30)
  { r<-seq(low, hi, length=n)
    z<-outer (r,r,f)
    persp(z)
  }

###############  EN R ###############
function.draw<-
function (f, low=-1, hi=1, n=30)
  { r<-seq(low, hi, length=n)
    z<-outer (r,r,f)
    persp(r,r,z,xlab="x")
  }