Table 1.

The four indicators of connectivity.

Indicators	Equation
Degree Centrality	${\displaystyle {\displaystyle \sum }_{i=1}^g}{x}_{ij}, i\ne j$ ${{\displaystyle \sum }}_{i=1}^g{x}_{ij}$: Number of connections node i has with (g − 1) nodes.
Closeness Centrality	$\frac{1}{{{\displaystyle \sum }}_{i=1}^{g}d\left(N_i,N_j\right)}$ ${{\displaystyle \sum }}_{i=1}^{g}d\left(N_i,N_j\right)$: The sum of the closest distance between node i and j g: number of nodes
Betweenness Centrality	${\displaystyle {\displaystyle \sum }_{i<k}}\frac{g_{jk} \left(N_i\right)}{g_{ik}}$ $g_{jk}$: the number of the closest routes between nodes j and k $g_{jk}\left(N_i\right)$: the number of closest routes between nodes i and j that includes node i
Eigenvector Centrality	$\lambda {\displaystyle {\displaystyle \sum }_i^g}{x}_{ij}{C}_E\left(N_j\right), i\ne j$ $\lambda$: Eigenvalue $g$: number of nodes $x$: the quantitative value of the connection between nodes i and j