where the factor 2 mc is arbitrary an it is just there for normalization. By doing carrying out the product, one find that there is not solution if A , A ′ , B i , B i ′ , C , C ′ are one dimensional constants. The lowest dimension where there is a solution is 4. Then A , A ′ , B , B ′ , C , C ′ are matrices that must satisfy the following relations: