[seqfan] Re: Q: unlabeled 2-regular digraphs

[Brendan McKay]

There are also A219894<https://oeis.org/A219894> (to which I just added two terms) and A219889.

Those two don't require connectivity, and the sequences that do require
connectivity seem to be missing.

Brendan.

On 12/3/19 9:40 pm, Brendan McKay wrote:

I just added  A306827,
Isomorphism classes of connected 2-in 2-out digraphs, allowing parallel
edges but not loops.

Someone might like to add the derived sequence without the connectivity
requirement.

Brendan.

On 12/3/19 8:18 pm, Richard J. Mathar wrote:


A007107 counts the labeled 2-regular digraphs with n nodes;
in A007108 we have the connected ones, in A001499 admitting loops.
Does the OEIS show somewhere the graphs without node labels, I mean the
digraphs where indegree=outdegree=2?

The natural variants of such a sequence would be:
  2-regular digraphs connected, loopless
  2-regular digraphs, loopless, not necessarily connected
  2-regular digraphs connected, loops allowed
  2-regular digraphs, loops allowed, not necessarily connected

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