in fact, the output quite telling:
sage:
g1=Graph({0:[1,4],1:[0,2,4],2:[1,3,4],3:[2],4:[0,1,2]});g2=Graph({0:[1],1:[0,2,4],2:[1,3,4],3:[2,4],4:[1,2,3]})
sage: g1.is_prime()
/home/dima/Sage/sage-dev/local/lib/python2.7/site-packages/sage/graphs/graph.py:6842:
********************************************************************************
Graph.modular_decomposition is known to return wrong results
This issue is being tracked at
http://trac.sagemath.org/sage_trac/ticket/13744.
********************************************************************************
True
sage: g2.is_prime()
False
On Sunday, December 11, 2016 at 9:45:14 PM UTC, Dima Pasechnik wrote:
>
> Sooner or later it was bound to happen; this function relies on
> modular_decomposition optional package,
> which is very iffy, and ought to be removed or redone, see
> https://trac.sagemath.org/ticket/17950
>
> Should we do something about it?
>
> @Harald - you can see the sender email by doing show source on the
> message; I put it into cc to this message.
>
>
> On Sunday, December 11, 2016 at 9:08:50 PM UTC, Harald Schilly wrote:
>>
>> This came in via the trac account request. I don't know the sender's
>> email address ...
>>
>> -- h
>>
>>
>> ---------- Forwarded message ----------
>> From: 'Jamel Dammak' via sage-trac-account <[email protected]>
>>
>> Date: Sun, Dec 11, 2016 at 10:01 PM
>> Subject: [sage-trac-account] Corrections about the function "is_prime"
>> for graphs
>> To: "[email protected]" <[email protected]>
>>
>>
>> Dear sir,
>>
>> there is an error concerning the function "is_prime" for graphs in
>> Sagemath, section Graph Theory.
>>
>> Here is a conter-example.
>>
>> g1=Graph({0:[1,4],1:[0,2,4],2:[1,3,4],3:[2],4:[0,1,2]});g2=Graph({0:[1],1:[0,2,4],2:[1,3,4],3:[2,4],4:[1,2,3]})
>>
>>
>> #g1.show();g2.show();
>> #[1,4] is a module of g1 so g1 is not prime
>> g2.is_isomorphic(g1), g1.is_prime(),g2.is_prime()
>>
>> This is a conter-example with two isomorphic graphs g1 and g2 on 5
>> vertices giving two different results
>> with the function "is prime".
>>
>> I have an algorithm giving the list of all prime graphs on at most 8
>> vertices.
>>
>> Best regards.
>>
>> --
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