block_and_cut_vertices is inconsistent when graph has one vertex

[jasongrout]

Currently, block_and_cut_vertices says that the vertex in a single-vertex graph is a cut vertex:

sage: Graph(1).blocks_and_cut_vertices()
([0], [0])

According to the definition of cut vertices given in the documentation of the function, a cut vertex, when removed, increases the connected components of the graph. Either that documentation should be changed to mention a corner case, or (preferably), the above computation should look like:

sage: Graph(1).blocks_and_cut_vertices()
([0], [])

CC: @sagetrac-hartke @rlmill @nathanncohen

Component: graph theory

Author: Nathann Cohen

Reviewer: Robert Miller

Merged: sage-4.3.1.alpha2

Issue created by migration from https://trac.sagemath.org/ticket/7853

[nathanncohen]

comment:1

Here it is ! Actually the answer [0],[0] seemed deliberate in the code, but my tests with the patch applied did not show any error coming from the fix :-)

( Apply on top of patches from #7634 )

Nathann

[nathanncohen]

Apply on top of patches from #7634

[rlmill]

Author: Nathann Cohen

[rlmill]

Reviewer: Robert Miller

[rlmill]

comment:2

Attachment: trac_7853.patch.gz

Looks good to me.

[rlmill]

Merged: 4.3.1.alpha2

[sagetrac-mvngu]

Changed merged from 4.3.1.alpha2 to sage-4.3.1.alpha2