doc: eigenvalue concentration example

examples/eigenvalue_concentration.py

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+r"""
+Concentration of the eigenvalues
+================================
+
+The eigenvalues of the graph Laplacian concentrates to the same value as the
+graph becomes full.
+"""
+
+from matplotlib import pyplot as plt
+import pygsp as pg
+
+n_neighbors = [1, 2, 5, 8]
+fig, axes = plt.subplots(4, len(n_neighbors), figsize=(15, 10))
+
+for k, ax in zip(n_neighbors, axes.T):
+    graph = pg.graphs.Ring(17, k=k)
+    graph.compute_fourier_basis()
+    graph.plot(graph.U[:, 1], ax=ax[0])
+    ax[0].axis('equal')
+    ax[1].spy(graph.W)
+    ax[2].plot(graph.e, '.')
+    ax[2].set_title('k={}'.format(k))
+    graph.set_coordinates('line1D')
+    graph.plot(graph.U[:, :4], ax=ax[3], title='')
+
+fig.tight_layout()