1 | initial version |

Define the graph `G`

and plot it:

```
sage: G = Graph([(1, 2), (2, 3), (3, 4), (3, 5), (2, 5), (5, 6)]); G
Graph on 6 vertices
sage: G.plot()
Launched png viewer for Graphics object consisting of 13 graphics primitives
```

Find adjacency matrix `b`

of `G`

, and inverse `u`

of `b`

:

```
sage: b = G.adjacency_matrix()
sage: u = ~b
sage: b, u
(
[0 1 0 0 0 0] [ 0 1 0 -1 0 -1]
[1 0 1 0 1 0] [ 1 0 0 0 0 0]
[0 1 0 1 1 0] [ 0 0 0 1 0 0]
[0 0 1 0 0 0] [-1 0 1 0 0 -1]
[0 1 1 0 0 1] [ 0 0 0 0 0 1]
[0 0 0 0 1 0], [-1 0 0 -1 1 0]
)
```

Some entries in `u`

are `-1`

, so it's slightly weird to use it
as adjacency matrix, but it works, with edges for nonzero entries.

```
sage: H = Graph(u)
sage: H.plot()
Launched png viewer for Graphics object consisting of 13 graphics primitives
```

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