This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A242278 #63 Jul 04 2023 11:42:06
%S A242278 0,6,18,72,216,702,2106,6480,19440,58806,176418,530712,1592136,
%T A242278 4780782,14342346,43040160,129120480,387400806,1162202418,3486725352,
%U A242278 10460176056,31380882462,94142647386,282429005040,847287015120,2541864234006,7625592702018,22876787671992
%N A242278 Number of non-palindromic n-tuples of 3 distinct elements.
%H A242278 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3, 3, -9).
%F A242278 a(n) = 1/2 * 3^(n/2) * ((sqrt(3)-1)*(-1)^n - sqrt(3)-1) + 3^n.
%F A242278 a(n) = 3^n - 3^ceiling(n/2).
%F A242278 a(n) = A000244(n) - A056449(n).
%F A242278 G.f.: (6*x) / (1 - 3*x - 3*x^2 + 9*x^3).
%F A242278 a(n) = 6*A167993(n). [_Bruno Berselli_, Aug 19 2014]
%e A242278 For n=3, the a(3)=18 solutions (non-palindromic 3-tuples) are:
%e A242278 {0,0,1}, {0,0,2}, {0,1,1}, {0,1,2}, {0,2,1}, {0,2,2}, {1,0,0}, {1,0,2},
%e A242278 {1,1,0}, {1,1,2}, {1,2,0}, {1,2,2}, {2,0,0}, {2,0,1}, {2,1,0}, {2,1,1},
%e A242278 {2,2,0}, {2,2,1}.
%p A242278 A242278:=n->(1/2)* 3^(n/2) * ((sqrt(3)-1) * (-1)^n - sqrt(3)-1) + 3^n: seq(A242278(n), n=1..28); # _Wesley Ivan Hurt_, Aug 17 2014.
%t A242278 Table[1/2 * 3^(n/2) * ((Sqrt(3)-1) * (-1)^n - Sqrt(3)-1) + 3^n, {n, 28}]
%o A242278 (PARI) a(n)=3^n-3^ceil(n/2) \\ _Charles R Greathouse IV_, Dec 10 2014
%Y A242278 Cf. A167993, A233411, A242026, A240437.
%K A242278 nonn,easy
%O A242278 1,2
%A A242278 _Mikk Heidemaa_, Aug 16 2014