I'm watching a video on smoothed particle hydrodynamics it just blindly claims that these smoothing kernels are pretty good.

$$W(r-r_b,h)\equiv\dfrac{315}{64\pi h^9}\left(h^2-|r-r_b|^2\right)^3$$ $$\nabla W(r-r_b,h)\equiv\dfrac{-45}{\pi h^6}\left(h-|r-r_b|\right)^2\left(\dfrac{r-r_b}{|r-r_b|}\right)$$ $$\nabla^2W\equiv\dfrac{45}{\pi h^6}\left(h-|r-r_b|\right)$$

However, it provides no citation, nor does it explain how each of these terms are derived.

Where do these come from?