x, r { \frac{v}{|v|}}\right ) { f ^{\mathrm{in}}}(x,v)\,{ \mathrm{d}(x,v)}} \\ &= { \int _{{{{\bf}R}}^d \times {{{\bf}R}}^d}\!\!\! \eta (0) \varphi (x,v) { \left \langle { f ^{\mathrm{in}}}\right \rangle }(x,v)\,{ \mathrm{d}(x,v)}}