OFFSET
1,1
COMMENTS
Note that 4 is the smallest value of A177961.
LINKS
FORMULA
a(n+2) = a(n)+15.
a(n) == (-1)^n (mod 3).
a(n) = 15*(n/2-1/4)+7*(-1)^n/4. - R. J. Mathar, Oct 25 2010
k such that k == 2 or -2 (mod 15). - Robert Israel, Jul 31 2015
Sum_{n>=1} (-1)^(n+1)/a(n) = tan(3*Pi/10)*Pi/15 = sqrt(1+2/sqrt(5))*Pi/15. - Amiram Eldar, Feb 28 2023
From Amiram Eldar, Nov 25 2024: (Start)
Product_{n>=1} (1 - (-1)^n/a(n)) = cos(3*Pi/10)*sec(11*Pi/30).
Product_{n>=1} (1 + (-1)^n/a(n)) = sec(Pi/15)/2. (End)
MAPLE
seq(seq(15*i+j, j=[2, 13]), i=0..100); # Robert Israel, Jul 31 2015
MATHEMATICA
Table[15 (n/2 - 1/4) + 7 (-1)^n/4, {n, 60}] (* Vincenzo Librandi, Aug 01 2015 *)
LinearRecurrence[{1, 1, -1}, {2, 13, 17}, 80] (* Harvey P. Dale, Nov 01 2023 *)
PROG
(Magma) [15*(n/2-1/4)+7*(-1)^n/4: n in [1..60]]; // Vincenzo Librandi, Aug 01 2015
CROSSREFS
KEYWORD
nonn,easy,changed
AUTHOR
Vladimir Shevelev, May 16 2010
EXTENSIONS
More terms from R. J. Mathar, Oct 25 2010
STATUS
approved