https://github.com/Xingyixie/Social_network_analysis/blob/master/work.with.pdf https://github.com/Xingyixie/Social_network_analysis/blob/05996b00b11e091c15d44c16718811867d2d8e49/get.on.with.pdf https://github.com/Xingyixie/Social_network_analysis/blob/05996b00b11e091c15d44c16718811867d2d8e49/best.friends.pdf