On the real vector space G = R 4 {\displaystyle {}G=\mathbb {R} ^{4}} of mulled wines, we consider the two linear maps
and
We consider π {\displaystyle {}\pi } as the price function, and κ {\displaystyle {}\kappa } as the caloric function. Determine a basis for ker ( π ) {\displaystyle {}\operatorname {ker} {\left(\pi \right)}} , one for ker ( κ ) {\displaystyle {}\operatorname {ker} {\left(\kappa \right)}} and one for ker ( π × κ ) {\displaystyle {}\operatorname {ker} {\left(\pi \times \kappa \right)}} .