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P0001. Let $D$ be a closed unit circle, centered at the origin. The function $f:D\to\bbR$ is a second-order continuously differentiable convex function, and $f\geq 0$ is at $\p D$. Try Certificate: $$\bex f(0)\geq-\f{1}{\sqrt{\pi}}\sex{\iint_D (f_{xx}f_{yy}-f_{xy}^2)\rd x\rd y}^\f{1}{2}. \eex$$