def pa(graph):
"""
Calculates Preferential Attachment index.
Returns S the similarity matrix.
"""
A = gts.adjacency(graph)
S = np.zeros(A.shape)
for i in xrange(S.shape[0]):
for j in xrange(S.shape[0]):
i_degree = graph.vertex(i).out_degree()
j_degree = graph.vertex(j).out_degree()
factor = i_degree * j_degree
S[i,j] = factor
return S