OFFSET
0,3
COMMENTS
For n >= 1, a(n) is also the number of hydronitrogen molecules containing only n nitrogen trivalent (octet rule satisfying) atoms. So for example, diazene is counted but hydrazoic acid is not because the former has only trivalent nitrogens and the latter has two non-trivalent nitrogens.
Some of the molecules are theoretical and may or may not exist due to their strained geometries.
Apparently the same as A243391 for n > 2. - Georg Fischer, Oct 16 2018
This is the case since A243391 gives the number of loopless multigraphs with nodes of degree 3 or less. The extra graph in A243391 is the 3-regular graph on 2 nodes. - Andrew Howroyd, Mar 20 2020
FORMULA
a(n) = A243391(n) for n > 2. - Andrew Howroyd, Mar 20 2020
EXAMPLE
a(3) = 4 because there are 4 molecules satisfying the above condition: triazane, triazene, triazirine, triazidirine.
Note: hydrazoic acid is not counted because there are 2 nitrogens not satisfying the octet rule (one has a positive formal charge and the other one has a negative one).
Graphically, a(3) = 4 because there are 4 graphs satisfying the above condition: the linear graph, the linear graph with one double edge, the triangle graph, and the triangle graph with one double edge. - Michael B. Porter, Apr 28 2018
PROG
(nauty/shell) for n in {1..18}; do geng -c -D3 ${n} -q | multig -m2 -D3 -u; done
CROSSREFS
KEYWORD
nonn
AUTHOR
Natan Arie Consigli, Apr 17 2018
EXTENSIONS
a(20)-a(28) from Andrew Howroyd, Mar 20 2020
STATUS
approved