拉普拉斯矩阵对研究图论之所以重要，是因为可以用其特征值来估计图的诸多不变量，如连通度、直径、带宽等等。

The study of Laplacian matrix is important for graphs 'study because we can estimate many invariants of g, such as connectedness, diameter, bandwidth.

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研究了图的正规拉普拉斯矩阵特征值与图的坚韧度，并给出了它们之间的不等式关系。

In this paper, the eigenvalues of the normalized Laplacian and the toughness of graphs are studied and the relation of inequality between of them is given.

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