A134818 - OEIS

A134818

Number of unlabeled connected loopless multigraphs with n nodes of degree 4 or less and with at most triple edges.

1, 3, 9, 37, 146, 772, 4449, 30307, 228605, 1921464, 17652327, 176162548, 1893738334, 21806975279, 267636988052, 3486370839295, 48029272657002, 697542580286159, 10649954607360119, 170508064788069346, 2856122791685125616, 49951625299057923405

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

From Natan Arie Consigli, May 29 2017: (Start)

Original name was "Number of hydrocarbon structures that can be drawn (excluding stereoisomers)" but this has been replaced with a mathematical definition which is more consistent with the terms of the sequence and the program.

In chemical terms this counts the following, given n carbon atoms:

- carbon allotropes;

- aliphatic hydrocarbons;

- resonance structures of graphically non-equivalent anti-aromatic and aromatic hydrocarbons.

Some molecules are theoretical and may or may not exist.

(End)

Computed over a period of several years and confirmed using the Molgen program.

Terms for n = 8,9,10 calculated using an exhaustive algorithm and Nauty. The algorithm correctly found the 7 known terms and the known acyclic hydrocarbons (up to n=10, see A002986) were extracted from the results correctly. - Vesa Linja-aho (vesa.linja-aho(AT)tkk.fi), Apr 17 2008

Except for a(2), the same as A289157. The extra graph in A289157 is the 4-regular graph on 2 nodes. - Andrew Howroyd, Mar 20 2020

LINKS

Table of n, a(n) for n=1..22.

Brendan McKay, Nauty

Molgen, Publications

FORMULA

a(n) = A289157(n) for n > 2. - Andrew Howroyd, Mar 20 2020

EXAMPLE

For n = 2 there are a(2) = 3 structures that can be drawn with 2 carbons (ethane, ethene, and ethyne).

For n = 7 there are a(7) = 4449 structures that can be drawn with 7 carbons.

PROG

(nauty/bash) geng -c -D4 ${n} -q | multig -m3 -D4 -u