Combining the graph's corresponding adjacency matrix, we find a necessary condition by making use of rank of matrix and congruence of matrices. At last, we give an theory algorithm for judging graphs' isomorphism.%结合图对应的邻接矩阵，利用矩阵的秩和矩阵的合同关系，得到了图同构的一个必要条件；然后给出了图同构的一个理论判断的算法．
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