The second order Taylor approximation for a function f(mathbfx^*) is presented as f(mathbfx^* + h mathbfy) = f(mathbfx^*) + h nabla f(mathbfx^*)ᵗ mathbfy + dfrac12 h² mathbfyᵗ nabla² f(mathbfx^*) mathbfy + O(h³). But shouldn't there be a Hessian matrix in the third term? Or is the Laplacian in the third term somehow the Hessian? Can someone please clarify this?