= Quadratic four-direction box spline
{title2=$\Xi=(e_1,e_2,e_1+e_2,e_1-e_2)$}
This <box spline> is the density of the image of the uniform unit four-cube under the indicated direction <matrix>. Its <polynomial> pieces have total degree $4-2=2$. Removing three directions is necessary to leave a nonspanning set, giving $C^1$ <continuity>. Its knot arrangement is a criss-cross triangulation of a square lattice. Its refinement mask is $\frac14(1+z)(1+w)(1+zw)(1+z/w)$, up to the monomial that fixes the origin.
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