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:with <strong>I</strong><sub>d</sub> being the ''d × d'' [[identity matrix]], with ''m''<sub>2</sub> = 1 for the normal kernel
* D<sup>2</sup>''ƒ'' is the ''d × d'' Hessian matrix of second order partial derivatives of ''ƒ''
* <math>\bold{\Psi}_4 = \int (\operatorname{vec} \, \operatorname{D}^2 f(\bold{x})) (\operatorname{vec}^T \operatorname{D}^2 f(\bold{x})) \, d\bold{x}</math> is a ''d''<sup>2</sup> × ''d''<sup>2</sup> matrix of integrated fourth order partial derivatives of ''ƒ''
* vec is the vector operator which stacks the columns of a matrix into a single vector e.g. <math>\operatorname{vec}\begin{bmatrix}a & c \\ b & d\end{bmatrix} = \begin{bmatrix}a & b & c & d\end{bmatrix}^T.</math>
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