|\Phi(f)|=|\int_{-1}^1 f(t)\cdot \frac{1}{\sqrt[3]{1-t^2}} \,dt|\leq 
(\int_{-1}^1 |f(t)|^2 \,dt)^\frac{1}{2} (\int_{-1}^1 \frac{dt}{(1-t^2)^{2/3}})^\frac{1}{2}