login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A109110 a(n) = 2a(n-1) + a(n-2) - a(n-3); a(0)=4, a(1)=9, a(2)=20. 0
4, 9, 20, 45, 101, 227, 510, 1146, 2575, 5786, 13001, 29213, 65641, 147494, 331416, 744685, 1673292, 3759853, 8448313, 18983187, 42654834, 95844542, 215360731, 483911170, 1087338529, 2443227497, 5489882353, 12335653674, 27717962204 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Kekulé numbers for certain benzenoids.

REFERENCES

S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (pp. 286, 288, K{S(n)})

LINKS

Table of n, a(n) for n=0..28.

Index entries for linear recurrences with constant coefficients, signature (2,1,-1)

FORMULA

G.f.: (4 + z - 2z^2)/(1 - 2z - z^2 + z^3).

a(n) = A052534(n+2). - R. J. Mathar, Feb 03 2014

MAPLE

a[0]:=4:a[1]:=9:a[2]:=20: for n from 3 to 32 do a[n]:=2*a[n-1]+a[n-2]-a[n-3] od: seq(a[n], n=0..32);

CROSSREFS

Sequence in context: A020708 A144686 A144685 * A108870 A331942 A111587

Adjacent sequences:  A109107 A109108 A109109 * A109111 A109112 A109113

KEYWORD

nonn,easy

AUTHOR

Emeric Deutsch, Jun 19 2005

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 31 00:29 EDT 2020. Contains 334747 sequences. (Running on oeis4.)