OFFSET
6,2
COMMENTS
Here 'alpha' is the number of atoms the two rings have in common.
Equivalently, the number of connected graphs on n unlabeled nodes with exactly 2 cycles without any shared node and all nodes having degree at most 4. See A121162 for the special case of the two cycles having the same length. - Andrew Howroyd, May 25 2018
LINKS
Andrew Howroyd, Table of n, a(n) for n = 6..200
J. B. Hendrickson and C. A. Parks, Generation and Enumeration of Carbon skeletons, J. Chem. Inf. Comput. Sci., 31 (1991), pp. 101-107. See Table VII column 2 on page 104.
EXAMPLE
If n=6 then the number of bicyclics when 'alpha' = zero is 1.
If n=7 then the number of bicyclics when 'alpha' = zero is 4.
If n=8 then the number of bicyclics when 'alpha' = zero is 20.
If n=9 then the number of bicyclics when 'alpha' = zero is 76.
From Andrew Howroyd, May 25 2018: (Start)
Case n=7: illustrations of the 4 graphs:
o o o o o o o o---o o o---o
/ \ / \ / \ / \ / / \ / \ / \ / \ \ \
o---o o---o o---o---o---o o---o---o---o o---o---o---o
(End)
PROG
(PARI) \\ here G is A000598 as series
G(n)={my(g=O(x)); for(n=1, n, g = 1 + x*(g^3/6 + subst(g, x, x^2)*g/2 + subst(g, x, x^3)/3) + O(x^n)); g}
C1(n)={sum(i=1, n\2-1, sum(j=1, n\2-i, (d1^(2*(i+j)) + 2*d1^(2*i)*d2^j + d2^(i+j))*(1 + d1)^2))/(8*(1-d1))}
C2(n)={sum(k=1, n\4, 2*(d2^(2*k) + d4^k)*(1 + d2))*(1+d1)/(8*(1-d2))}
seq(n)={my(s=G(n)); my(d=x*(s^2+subst(s, x, x^2))/2); my(g(p, e)=subst(p + O(x*x^(n\e)), x, x^e)); Vec(O(x^n/x) + g(s, 1)^2*substvec(C1(n-2), [d1, d2], [g(d, 1), g(d, 2)]) + g(s, 2)*substvec(C2(n-2), [d1, d2, d4], [g(d, 1), g(d, 2), g(d, 4)]))} \\ Andrew Howroyd, May 25 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Parthasarathy Nambi, Jan 29 2007
EXTENSIONS
Terms a(16) and beyond from Andrew Howroyd, May 25 2018
STATUS
approved