Voozh

A056081

Numbers that are congruent to {1, 26} mod 27.

1, 26, 28, 53, 55, 80, 82, 107, 109, 134, 136, 161, 163, 188, 190, 215, 217, 242, 244, 269, 271, 296, 298, 323, 325, 350, 352, 377, 379, 404, 406, 431, 433, 458, 460, 485, 487, 512, 514, 539, 541, 566, 568, 593, 595, 620, 622, 647, 649, 674, 676, 701, 703

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Numbers k such that k^2 == 1 (mod 3^3).

LINKS

Table of n, a(n) for n=1..53.

Index entries for linear recurrences with constant coefficients, signature (1,1,-1).

FORMULA

a(n) = 27*(n-1)-a(n-1) with n>1, a(1) = 1. - Vincenzo Librandi, Aug 07 2010

G.f.: x*(1+25*x+x^2) / ( (1+x)*(x-1)^2 ). - R. J. Mathar, Jul 10 2015

Sum_{n>=1} (-1)^(n+1)/a(n) = cot(Pi/27)*Pi/27. - Amiram Eldar, Dec 31 2021