OFFSET
0,2
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).
FORMULA
G.f.: 8*x*(1 + 5*x + 5*x^2 + x^3)/(1 - x)^5. - Colin Barker, Jan 08 2012
E.g.f.: 4*x*(2 + 8*x + 6*x^2 + x^3)*exp(x). - Michael Somos, Jul 05 2018
a(n) = a(-n) = (2*n)^2 + (2*n^2)^2 = (2*n^2 + 1)^2 - 1. - Michael Somos, Jul 05 2018
From Amiram Eldar, Oct 25 2024: (Start)
Sum_{n>=1} 1/a(n) = Pi^2/24 + (1-Pi*coth(Pi))/8.
Sum_{n>=1} (-1)^(n+1)/a(n) = Pi^2/48 + (Pi*cosech(Pi)-1)/8. (End)
EXAMPLE
G.f. = 8*x + 80*x^2 + 360*x^3 + 1088*x^4 + 2600*x^5 + 5328*x^6 + 9800*x^7 + ... - Michael Somos, Jul 05 2018
MATHEMATICA
Table[4*n^2*(n^2 + 1), {n, 0, 30}] (* OR *)
Table[Round[N[Sinh[2 ArcSinh[n]]^2, 100]], {n, 0, 30}]
a[ n_] := TrigExpand @ Sinh[ 2 ArcSinh @ n]^2; (* Michael Somos, Jul 05 2018 *)
PROG
(Magma) [4*n^2*(n^2+1): n in [0..40]]; // Vincenzo Librandi, Jun 15 2011
(PARI) a(n)=4*n^2*(n^2+1) \\ Charles R Greathouse IV, Apr 17 2012
(PARI) a(n)=8*binomial(n^2+1, 2) \\ Charles R Greathouse IV, Apr 17 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Artur Jasinski, Feb 10 2010
EXTENSIONS
Name corrected by Jianing Song, Nov 23 2018
STATUS
approved