= Tensor-product surface basis
{title2=$b_{jk}(u,v)=\phi_j(u)\psi_k(v)$}
Products of two univariate bases form a surface basis. Nonnegativity, <partition of unity> and coordinate-wise <linear precision> pass to the products by multiplication of the respective univariate identities. If the factors are $C^r$ and $C^s$, each mixed <derivative> through orders $r,s$ is the product of the corresponding <derivatives>, giving continuous mixed <derivatives> and joint $C^{\min(r,s)}$ regularity.
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