x0 <- 1.9
y0 <- -0.3
r <- 0.5

f <- function(x,y)
  x^3-4*x*y^2-2*x^2*y-3*x*y+2*y^3+3*x

x <- seq(x0-r,x0+r,r/100)
y <- seq(y0-r,y0+r,r/100)
z <- outer(x,y,f)

t <- seq(0,2*pi,0.01)
contour(x,y,z,asp=1,nlevels=20)
lines(2*cos(t),sin(t),col="red")

locator(1)
f(2,-0.25)

x0 <- -0.5
y0 <- 1
r <- 0.5

f <- function(x,y)
  x^3-4*x*y^2-2*x^2*y-3*x*y+2*y^3+3*x

x <- seq(x0-r,x0+r,r/100)
y <- seq(y0-r,y0+r,r/100)
z <- outer(x,y,f)

t <- seq(0,2*pi,0.01)
contour(x,y,z,asp=1,nlevels=20)
lines(2*cos(t),sin(t),col="red")

locator(1)
f(-0.4,1)

x0 <- 1.9
y0 <- -0.3
r <- 0.5

f <- function(x,y)
  x^3-4*x*y^2-2*x^2*y-3*x*y+2*y^3+3*x

x <- seq(x0-r,x0+r,r/100)
y <- seq(y0-r,y0+r,r/100)
z <- outer(x,y,f)

t <- seq(0,2*pi,0.01)
contour(x,y,z,asp=1,nlevels=20)
lines(2*cos(t),sin(t),col="red")

locator(1)
f(2,-0.25)