Equational Theories

Let Di = (Vi,i?i), . . . ,D n = (Vn,E n) be graphs. The direct product D = D\ x • • • Dn is the graph D = (V, E ), where V = V\ x • • • x Vn and

E = { ( (^ i , . .. ,xn)(yi , . .. ,2/n)) | {xuVi) € Ei for i = 1,.. . ,n}.