### linear-algebra

#### networkx can't calculate algebraic connectivity

I can compute the algebraic connectivity of the complete graph on 20 vertices in fraction of a second using import networkx D = {} for i in range(20): D[i] = [j for j in range(20)] G = networkx.Graph(D) networkx.algebraic_connectivity(G) However, in a process I generate a graph (on 20 nodes) that I ask networkx to compute its algebraic connectivity, and it keeps running for ever with no errors. Here is the graph: import networkx D = {0: [32, 33, 19, 5, 21, 37, 6, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 5: [32, 0, 33, 19, 37, 21, 6, 22, 38, 39, 41, 26, 42, 11, 43, 44, 28, 15, 31], 6: [0, 32, 33, 19, 5, 37, 21, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 11: [32, 0, 33, 19, 21, 37, 5, 6, 22, 38, 39, 41, 26, 42, 43, 28, 44, 15, 31], 15: [0, 32, 33, 19, 5, 21, 37, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 31], 19: [0, 32, 33, 5, 21, 37, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 21: [32, 0, 33, 19, 37, 5, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 22: [32, 33, 19, 5, 21, 37, 6, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 26: [0, 32, 33, 19, 5, 21, 37, 6, 22, 38, 39, 41, 42, 11, 43, 28, 44, 15, 31], 28: [32, 0, 33, 19, 21, 37, 5, 6, 22, 38, 39, 41, 26, 42, 11, 43, 44, 15, 31], 31: [32, 0, 33, 19, 5, 21, 37, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15], 32: [0, 33, 19, 5, 21, 37, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 31, 15], 33: [0, 32, 19, 5, 21, 37, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 37: [32, 0, 33, 19, 5, 21, 6, 22, 38, 39, 41, 26, 42, 11, 43, 28, 44, 31, 15], 38: [32, 0, 33, 19, 21, 37, 5, 6, 22, 39, 41, 26, 42, 11, 43, 28, 44, 15, 31], 39: [0, 32, 33, 19, 5, 21, 37, 6, 22, 38, 41, 26, 42, 11, 43, 28, 44, 15, 31], 41: [32, 0, 33, 19, 21, 37, 5, 38, 6, 22, 39, 26, 42, 11, 43, 28, 44, 15, 31], 42: [32, 0, 33, 19, 21, 37, 5, 6, 22, 38, 39, 41, 26, 11, 43, 28, 44, 15, 31], 43: [32, 0, 33, 19, 21, 37, 5, 6, 22, 38, 39, 41, 26, 42, 11, 28, 44, 15, 31], 44: [32, 0, 33, 19, 5, 21, 37, 38, 6, 22, 39, 41, 42, 26, 11, 43, 28, 15, 31]} G = networkx.Graph(D) networkx.algebraic_connectivity(G) Any reasons why it is so, and how to fix it?

There seems to be a bug in the tracemin method that is the default with networkx.algebraic_connectivity(). Try using networkx.algebraic_connectivity(G, method='lanczos')

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