OFFSET
0,2
COMMENTS
With a different offset, number of n-permutations of 5 objects u, v, w, z, x with repetition allowed, containing exactly two u's. - Zerinvary Lajos, Dec 29 2007
Also convolution of A000302 with A002697, also convolution of A002457 with itself. - Rui Duarte, Oct 08 2011
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..400
Adam Ehrenberg, Joseph T. Iosue, Abhinav Deshpande, Dominik Hangleiter, and Alexey V. Gorshkov, The Second Moment of Hafnians in Gaussian Boson Sampling, arXiv:2403.13878 [quant-ph], 2024. See p. 30.
Index entries for linear recurrences with constant coefficients, signature (12,-48,64).
FORMULA
a(n) = (n+2)*(n+1)*2^(2*n-1).
G.f.: 1/(1-4*x)^3.
a(n) = Sum_{a+b+c+d+e+f=n} f(a)*f(b)*f(c)*f(d)*f(e)*f(f) with f(n)=A000984(n). - Philippe Deléham, Jan 22 2004
a(n) = binomial(n+2,n) * 4^n. - Rui Duarte, Oct 08 2011
E.g.f.: (1 + 8*x + 8*x^2)*exp(4*x). - G. C. Greubel, Jul 20 2019
From Amiram Eldar, Jan 05 2022: (Start)
Sum_{n>=0} 1/a(n) = 8 - 24*log(4/3).
Sum_{n>=0} (-1)^n/a(n) = 40*log(5/4) - 8. (End)
MAPLE
seq((n+2)*(n+1)*4^n/2, n=0..30); # Zerinvary Lajos, Apr 25 2007
MATHEMATICA
Table[4^n*Binomial[n+2, n], {n, 0, 30}] (* G. C. Greubel, Jul 20 2019 *)
PROG
(Sage) [4^(n-2)*binomial(n, 2) for n in range(2, 30)] # Zerinvary Lajos, Mar 11 2009
(Magma) [4^n*Binomial(n+2, 2): n in [0..30]]; // Vincenzo Librandi, Oct 15 2011
(PARI) a(n)=(n+2)*(n+1)<<(2*n-1) \\ Charles R Greathouse IV, Aug 21 2015
(GAP) List([0..30], n-> 4^n*Binomial(n+2, n) ); # G. C. Greubel, Jul 20 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
STATUS
approved