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%I #19 Dec 31 2021 05:48:25
%S 1,26,28,53,55,80,82,107,109,134,136,161,163,188,190,215,217,242,244,
%T 269,271,296,298,323,325,350,352,377,379,404,406,431,433,458,460,485,
%U 487,512,514,539,541,566,568,593,595,620,622,647,649,674,676,701,703
%N Numbers that are congruent to {1, 26} mod 27.
%C Numbers k such that k^2 == 1 (mod 3^3).
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F a(n) = 27*(n-1)-a(n-1) with n>1, a(1) = 1. - _Vincenzo Librandi_, Aug 07 2010
%F G.f.: x*(1+25*x+x^2) / ( (1+x)*(x-1)^2 ). - _R. J. Mathar_, Jul 10 2015
%F Sum_{n>=1} (-1)^(n+1)/a(n) = cot(Pi/27)*Pi/27. - _Amiram Eldar_, Dec 31 2021
%t x=3; Select[ Range[ 1000 ], PowerMod[ #, x-1, x^3 ]==1& ]
%K nonn,easy
%O 1,2
%A _Robert G. Wilson v_, Jun 08 2000
%E Definition rewritten by _Bruno Berselli_, Nov 19 2012