code: def eigenvalue(nodes, node=None):
      
      if node is None:
          return sorted([eigenvalue(nodes, n) for n in nodes])[0]  # return lowest eigenvalue
      else:
          return len([1 for n in nodes if hops(node, n)])
docs: calculate the eigenvalue (number of connections) for a given node in an array of nodes connected by an array of links
      if no node is given, return the minimum eigenvalue in the whole network