OFFSET
0,2
COMMENTS
Kekulé numbers for certain benzenoids.
LINKS
Bo Gyu Jeong, Table of n, a(n) for n = 0..5000
Somaya Barati, Beáta Bényi, Abbas Jafarzadeh, and Daniel Yaqubi, Mixed restricted Stirling numbers, arXiv:1812.02955 [math.CO], 2018.
S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (p. 230, no. 26).
C. Krishnamachari, The operator (xD)^n, J. Indian Math. Soc., 15 (1923),3-4. [Annotated scanned copy]
Index entries for linear recurrences with constant coefficients, signature (6,-15,20,-15,6,-1).
FORMULA
From Zerinvary Lajos, Jan 20 2007: (Start)
a(n) = (n+1)*Stirling2(n+3,n+1). (End)
From Colin Barker, Apr 22 2020: (Start)
G.f.: (1 + 8*x + 6*x^2) / (1 - x)^6.
a(n) = 6*a(n-1) - 15*a(n-2) + 20*a(n-3) - 15*a(n-4) + 6*a(n-5) - a(n-6) for n>5.
(End)
From Amiram Eldar, May 29 2022: (Start)
Sum_{n>=0} 1/a(n) 2*Pi^2 + 54*sqrt(3)*Pi/5 + 486*log(3)/5 - 921/5.
Sum_{n>=0} (-1)^n/a(n) = Pi^2 - 108*sqrt(3)*Pi/5 - 528*log(2)/5 + 909/5. (End)
E.g.f.: (1/24)*(24 +312*x +576*x^2 +304*x^3 +55*x^4 +3*x^5)*exp(x). - G. C. Greubel, Oct 19 2023
MAPLE
a:= n-> (n+1)^2*(n+2)*(n+3)*(3*n+4)/24: seq(a(n), n=0..36);
seq((n+1)*stirling2(n+3, n+1), n=0..32); # Zerinvary Lajos, Jan 20 2007
MATHEMATICA
Table[((n+1)^2 (n+2)(n+3)(3n+4))/24, {n, 0, 40}] (* or *) Table[n StirlingS2[n+2, n], {n, 40}] (* Harvey P. Dale, Dec 01 2013 *)
PROG
(PARI) Vec((1 + 8*x + 6*x^2) / (1 - x)^6 + O(x^30)) \\ Colin Barker, Apr 22 2020
(Magma) [(n+1)*StirlingSecond(n+3, n+1): n in [0..40]]; // G. C. Greubel, Oct 19 2023
(SageMath) [(n+1)*stirling_number2(n+3, n+1) for n in range(41)] # G. C. Greubel, Oct 19 2023
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch, Jun 13 2005
STATUS
approved