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A056081
Numbers that are congruent to {1, 26} mod 27.
1
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
OFFSET
1,2
COMMENTS
Numbers k such that k^2 == 1 (mod 3^3).
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
MATHEMATICA
x=3; Select[ Range[ 1000 ], PowerMod[ #, x-1, x^3 ]==1& ]
CROSSREFS
Sequence in context: A042401 A042402 A247394 * A081644 A256619 A055109
KEYWORD
nonn,easy
AUTHOR
Robert G. Wilson v, Jun 08 2000
EXTENSIONS
Definition rewritten by Bruno Berselli, Nov 19 2012
STATUS
approved