= Triweight kernel
{title2=$K(x)=\frac{35}{32}(1-x^2)^3\mathbf1_{|x|<1}$}
The triweight kernel is a compactly supported symmetric probability density with variance $1/9$. Its value and first two derivatives vanish at the support endpoints. Its integrated squared second derivative is $35$, and it minimizes that curvature among twice continuously differentiable densities with zero mean and the same variance, by the <triweight density minimizes integrated squared curvature> inequality.
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