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 A376478 #16 Mar 10 2025 11:02:30
%S A376478 1,2,3,9,27,81,243,729,2187,6561,19683,59049,177147,531441,1594323,
%T A376478 4782969,14348907,43046721,129140163,387420489,1162261467,3486784401,
%U A376478 10460353203,31381059609,94143178827,282429536481,847288609443,2541865828329,7625597484987,22876792454961
%N A376478 a(1) = 1, a(2) = 2, and a(n) = 3^(n-2) for n > 2.
%C A376478 Graham's conjecture: also numbers k such sigma(k) - k = floor(k/2). See Guy.
%C A376478 Also the domination number of the n-Sierpinski gasket graph. - _Eric W. Weisstein_, Mar 10 2025
%D A376478 R. K. Guy, Unsolved Problems in Number Theory, Springer, 1st edition, 1981. See section B2.
%H A376478 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/DominationNumber.html">Domination Number</a>.
%H A376478 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SierpinskiGasketGraph.html">Sierpinski Gasket Graph</a>.
%H A376478 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (3).
%F A376478 a(n) = 3*a(n-1) for n > 3.
%F A376478 G.f.: (1 - x - 3*x^2)/(1 - 3*x).
%F A376478 E.g.f.: (2 + exp(3*x) + 3*x)/3.
%t A376478 LinearRecurrence[{3},{1,2,3},30]
%o A376478 (Python)
%o A376478 def A376478(n): return n if n<3 else 3**(n-2) # _Chai Wah Wu_, Nov 13 2024
%Y A376478 Cf. A000244, A140429, A001065, A004526.
%K A376478 nonn,easy
%O A376478 1,2
%A A376478 _Stefano Spezia_, Sep 24 2024