OFFSET
1,1
LINKS
FORMULA
G.f.: 4*x*(1+x+x^2) / ( (1+x)*(x-1)^2 ).
a(n) = 4 * A001651(n).
Iff phi(n) = phi(3n/2), then n is in A069587. - Labos Elemer, Feb 25 2004
a(n) = 12*(n-1)-a(n-1) (with a(1)=4). - Vincenzo Librandi, Nov 16 2010
From Wesley Ivan Hurt, May 21 2016: (Start)
a(n) = a(n-1) + a(n-2) - a(n-3) for n>3.
a(n) = 6n - 3 - (-1)^n.
a(n) = -a(1-n), a(n) = A092899(n) + 1 for n>0. (End)
Sum_{n>=1} (-1)^(n+1)/a(n) = Pi*sqrt(3)/36. - Amiram Eldar, Dec 30 2021
MAPLE
MATHEMATICA
Table[6n-3-(-1)^n, {n, 80}] (* Wesley Ivan Hurt, May 21 2016 *)
LinearRecurrence[{1, 1, -1}, {4, 8, 16}, 60] (* Harvey P. Dale, Oct 07 2021 *)
PROG
(Magma) [n : n in [0..400] | n mod 12 in [4, 8]]; // Wesley Ivan Hurt, May 21 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Giovanni Teofilatto, Feb 19 2004
EXTENSIONS
Edited and extended by Ray Chandler, Feb 21 2004
STATUS
approved