2335.sagews

var('x,y')
f(x,y)= (x^2-y^2)/(x^2+y^2)
plot3d(f,(x,-0.1,0.1),(y,-0.1,0.1))

# The limit at (0,0) does not exist even though it appears to in this graph.
var('x,y')
g(x,y)= (x*y^2)/(x^2+y^4)
plot3d(g,(x,-0.001,0.001),(y,-0.001,0.001), color='red')

# Another limit at (0,0) that does not exist.  Check!
var('x,y')
h(x,y)= (x*y)/(x^2+y^2)
plot3d(h,(x,-0.001,0.001),(y,-0.001,0.001), color='green')