The variables *n*_{x} und *n*_{y} are the components of the normal vector along the boundary.

The integral expression from Eq. 3-16 is called “weak form” of Eq. 3-7. The reader should now recall the first section of this chapter (… flipping pages back might be necessary). If we have a look at Eq. 3-16, we notice that the order of the differential referring to the variable T is not m=2 but m=1 in the integral (reduction), but at the same time a differential for has been added (transport from T to ). Additionally, the natural boundary condition from Eq. 3-9 has been integrated into the weal form by means of the boundary integral.