Limit $\lim_{(x,y)\rightarrow (0,0)}\frac{xy^3}{x^2+y^4}$
Multivariable limit proof: $\lim\limits_{(x,y)\rightarrow (0,0)}\frac{\left|x\right|^a\left|y\right|^b}{\left|x\right|^c + \left|y\right|^d} = 0$