get rid of amsmath warning

curves/curves.tex

@@ -459,7 +459,7 @@ \subsection{Triangular B\'ezier surfaces}
 where the surface is described by:
 
 \begin{equation}
-S(u,v,w) = \sum_{i+j+k=n \atop i,j,k \geq 0} B^n_{i,j,k}(u,v,w) p_{i,j,k},
+S(u,v,w) = \sum_{\genfrac{}{}{0pt}{}{i+j+k=n}{i,j,k \geq 0}} B^n_{i,j,k}(u,v,w) p_{i,j,k},
 \end{equation}
 
 and \(B^n_{i,j,k}\) are the \emph{bivariate Bernstein polynomials}\marginnote{bivariate Bernstein polynomials}\index{bivariate Bernstein polynomials}, which are given by: