Graph.is_gallai_tree() method has an error in the code

[math1um]

The Graph.is_gallai_tree() method checks for whether a block's vertices induce a cycle tests if the number of edges is one more than the number of vertices. These should be the same.

The third-to-last-line currently has the test: gg.size() == len(c)+1

It should have the test: gg.size() == len(c)

A graph which is a 5-cycle with an appended edge to an external vertex is a gallai tree. But the existing method returns False. Call this graph "gg":

gg=graphs.CycleGraph(5)
gg.add_edge(0,5)
gg.is_gallai_tree() 

should return: True.

CC: @dcoudert

Component: graph theory

Author: Frédéric Chapoton

Branch/Commit: f8a9c73

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/25613

[fchapoton]

Commit: f8a9c73

[fchapoton]

Branch: public/25613

[fchapoton]

New commits:

f8a9c73

fixing is_gallai_tree

[fchapoton]

Author: Frédéric Chapoton

[tscrim]

comment:4

LGTM.

[tscrim]

Reviewer: Travis Scrimshaw

[vbraun]

Changed branch from public/25613 to f8a9c73