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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def vertex_neighbors(self):
 """ The vertex neighbors of each vertex of the mesh, determined from the cached vertex_adjacency_graph, if already existent. Returns vertex_neighbors : (len(self.vertices),) int Represents immediate neighbors of each vertex along the edge of a triangle Examples This is useful for getting nearby vertices for a given vertex, potentially for some simple smoothing techniques. [1,2,3,4] """

        graph = self.vertex_adjacency_graph
        neighbors = [list(graph.neighbors(i)) for
                     i in range(len(self.vertices))]
        return np.array(neighbors)