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# Source code for networkx.algorithms.centrality.second_order

# -*- coding: utf-8 -*-
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'''

import networkx as nx
from networkx.utils import not_implemented_for

# Authors: Erwan Le Merrer (erwan.lemerrer@technicolor.com)
''' Second order centrality measure.'''

__all__ = ['second_order_centrality']

[docs]@not_implemented_for('directed')
def second_order_centrality(G):
"""Compute the second order centrality for nodes of G.

The second order centrality of a given node is the standard deviation of
the return times to that node of a perpetual random walk on G:

Parameters
----------
G : graph
A NetworkX connected and undirected graph.

Returns
-------
nodes : dictionary
Dictionary keyed by node with second order centrality as the value.

Examples
--------
>>> G = nx.star_graph(10)
>>> soc = nx.second_order_centrality(G)
>>> print(sorted(soc.items(), key=lambda x:x[1])[0][0]) # pick first id
0

Raises
------
NetworkXException
If the graph G is empty, non connected or has negative weights.

--------
betweenness_centrality

Notes
-----
Lower values of second order centrality indicate higher centrality.

The algorithm is from Kermarrec, Le Merrer, Sericola and Trédan [1]_.

This code implements the analytical version of the algorithm, i.e.,
there is no simulation of a random walk process involved. The random walk
is here unbiased (corresponding to eq 6 of the paper [1]_), thus the
centrality values are the standard deviations for random walk return times
on the transformed input graph G (equal in-degree at each nodes by adding
self-loops).

Complexity of this implementation, made to run locally on a single machine,
is O(n^3), with n the size of G, which makes it viable only for small
graphs.

References
----------
.. [1] Anne-Marie Kermarrec, Erwan Le Merrer, Bruno Sericola, Gilles Trédan
"Second order centrality: Distributed assessment of nodes criticity in
complex networks", Elsevier Computer Communications 34(5):619-628, 2011.
"""

try:
import numpy as np
except ImportError:
raise ImportError('Requires NumPy: http://scipy.org/')

n = len(G)

if n == 0:
raise nx.NetworkXException("Empty graph.")
if not nx.is_connected(G):
raise nx.NetworkXException("Non connected graph.")
if any(d.get('weight', 0) < 0 for u, v, d in G.edges(data=True)):
raise nx.NetworkXException("Graph has negative edge weights.")

# balancing G for Metropolis-Hastings random walks
G = nx.DiGraph(G)
in_deg = dict(G.in_degree(weight='weight'))
d_max = max(in_deg.values())
for i, deg in in_deg.items():
if deg < d_max:

P = nx.to_numpy_matrix(G)
P = P / P.sum(axis=1)  # to transition probability matrix

def _Qj(P, j):
P = P.copy()
P[:, j] = 0
return P

M = np.empty([n, n])

for i in range(n):
M[:, i] = np.linalg.solve(np.identity(n) - _Qj(P, i),
np.ones([n, 1])[:, 0])  # eq 3

return dict(zip(G.nodes,
[np.sqrt((2*np.sum(M[:, i])-n*(n+1))) for i in range(n)]
))  # eq 6

# fixture for nose tests
def setup_module(module):
from nose import SkipTest
try:
import numpy
import scipy
except:
raise SkipTest("NumPy not available")