Given a set of elements X={x1, x2,... , xn} and n * n relationship matrix Rij, xi is associated with xj, Rij=1, on the contrary, Rij=0, A back branch (or limit) algorithm is designed, in X to find A loophole two sets A and B, satisfy: (1) A studying B=Φ; (2) each element of A and B all the elements in the associated; (3) B of each element is associated with all the elements in A; (4) A ∪ B total number of elements in the most,