The exact thin plate energy functional (TPEF) for a function f ( x , y ) {\displaystyle f(x,y)} is
∫ y 0 y 1 ∫ x 0 x 1 ( κ 1 2 + κ 2 2 ) g d x d y {\displaystyle \int _{y_{0}}^{y_{1}}\int _{x_{0}}^{x_{1}}(\kappa _{1}^{2}+\kappa _{2}^{2}){\sqrt {g}}\,dx\,dy}
where κ 1 {\displaystyle \kappa _{1}} and κ 2 {\displaystyle \kappa _{2}} are the principal curvatures of the surface mapping f {\displaystyle f} at the point ( x , y ) . {\displaystyle (x,y).} This is the surface integral of κ 1 2 + κ 2 2 , {\displaystyle \kappa _{1}^{2}+\kappa _{2}^{2},} hence the g {\displaystyle {\sqrt {g}}} in the integrand. Minimizing the exact thin plate energy functional would result in a system of non-linear equations. So in practice, an approximation that results in linear systems of equations is often used. The approximation is derived by assuming that the gradient of f {\displaystyle f} is 0. At any point where f x = f y = 0 , {\displaystyle f_{x}=f_{y}=0,} the first fundamental form g i j {\displaystyle g_{ij}} of the surface mapping f {\displaystyle f} is the identity matrix and the second fundamental form b i j {\displaystyle b_{ij}} is
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