One has

Denoting the jacobian

one finally has

When , the expression of the jacobian reduces to .

With a part of the boundary of a real element and the corresponding boundary on the reference element , one has

where is the unit normal to on . In a same way

For the unit normal to on .

Denoting

the matrix and

the matrix, then

and thus

In order to have uniform methods for the computation of elementary matrices, the Hessian is computed as a column vector whose components are . Then, with the matrix defined as

and the matrix defined as

one has

Assume one needs to compute the elementary “matrix”:

The computations to be made on the reference elements are

and

Those two tensor can be computed once on the whole reference element if the geometric transformation is linear (because is constant). If the geometric transformation is non-linear, what has to be stored is the value on each integration point. To compute the integral on the real element a certain number of reductions have to be made:

- Concerning the first term () nothing.
- Concerning the second term () a reduction with respect to with the matrix .
- Concerning the third term ()` a reduction of with respect to with the matrix and a reduction of with respect also to with the matrix

The reductions are to be made on each integration point if the geometric transformation is non-linear. Once those reductions are done, an addition of all the tensor resulting of those reductions is made (with a factor equal to the load of each integration point if the geometric transformation is non-linear).

If the finite element is non--equivalent, a supplementary reduction of the resulting tensor with the matrix has to be made.