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%I #20 Feb 27 2023 04:00:51
%S 1,18,20,37,39,56,58,75,77,94,96,113,115,132,134,151,153,170,172,189,
%T 191,208,210,227,229,246,248,265,267,284,286,303,305,322,324,341,343,
%U 360,362,379,381,398,400,417,419,436,438,455,457,474,476,493,495,512
%N Numbers k such that 18*k^2+1 is multiple of 19.
%C Numbers congruent to {1, 18} mod 19. - _Amiram Eldar_, Feb 27 2023
%H G. C. Greubel, <a href="/A113541/b113541.txt">Table of n, a(n) for n = 1..1000</a>
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F a(1)=1, a(2)=18 then add alternatively 2 and 17.
%F a(n) = 19*(n-1) - a(n-1), (with a(1)=1). - _Vincenzo Librandi_, Nov 13 2010
%F Sum_{n>=1} (-1)^(n+1)/a(n) = cot(Pi/19)*Pi/19. - _Amiram Eldar_, Feb 27 2023
%t s={};Do[If[IntegerQ[(18n^2+1)/19], AppendTo[s, n]], {n, 200}];s
%K nonn
%O 1,2
%A _Zak Seidov_, Jan 13 2006
%E a(22)-a(56) from _Vincenzo Librandi_, Oct 23 2009
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