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A122652 a(0)=0, a(1)=4; for n>1, a(n) = 10*a(n-1) - a(n-2). 3
0, 4, 40, 396, 3920, 38804, 384120, 3802396, 37639840, 372596004, 3688320200, 36510605996, 361417739760, 3577666791604, 35415250176280, 350574834971196, 3470333099535680, 34352756160385604, 340057228504320360, 3366219528882817996, 33322138060323859600 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Kekulé numbers for the benzenoids P_2(n).

a(n) are the values of m where A032528(m) - 1 has integer square roots. The roots are given by A001079. - Richard R. Forberg, Aug 05 2013

Numbers n such that 6*n^2 + 4 is a square. - Colin Barker, Mar 17 2014

REFERENCES

S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (p. 283, K{P_2(n)}).

LINKS

Table of n, a(n) for n=0..20.

John M. Campbell, An Integral Representation of Kekulé Numbers, and Double Integrals Related to Smarandache Sequences, arXiv preprint arXiv:1105.3399, 2011.

Tanya Khovanova, Recursive Sequences

Index entries for linear recurrences with constant coefficients, signature (10,-1).

FORMULA

a(n) = (1/6)*(5 + 2*sqrt(6))^n*sqrt(6) - (1/6)*sqrt(6)*(5 - 2*sqrt(6))^n, with n>=0. - Paolo P. Lava, Oct 02 2008

G.f.: 4*x/(1 - 10*x + x^2). - Philippe Deléham, Nov 17 2008

3*a(n)^2 + 2 = 2*A001079(n)^2. - Charlie Marion, Feb 01 2013

a(n) = (2*arcsinh(sqrt(2))*sinh(2*n*arcsinh(sqrt(2)))/log(sqrt(2) + sqrt(3)))/sqrt(6). - Artur Jasinski, Aug 09 2016

a(n) = 2*A001078(n). - Bruno Berselli, Nov 25 2016

MATHEMATICA

CoefficientList[Series[(4 z)/(z^2 - 10 z + 1), {z, 0, 200}], z] (* Vladimir Joseph Stephan Orlovsky, Jun 11 2011 *)

PROG

(PARI) a(n)=if(n<2, (n%2)*4, 10*a(n-1)-a(n-2)) \\ Benoit Cloitre, Sep 23 2006

CROSSREFS

Cf. A001078, A001079, A032528.

Sequence in context: A220310 A246152 A155641 * A093141 A220965 A278036

Adjacent sequences:  A122649 A122650 A122651 * A122653 A122654 A122655

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane, Sep 21 2006

EXTENSIONS

More terms and better definition from Benoit Cloitre, Sep 23 2006

STATUS

approved

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Last modified May 25 14:22 EDT 2017. Contains 287038 sequences.