Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #77 Aug 06 2024 02:13:58
%S 3,2,6,17,42,107,273,695,1770,4508,11481,29240,74469,189659,483027,
%T 1230182,3133050,7979309,20321850,51756059,131813277,335704463,
%U 854978262,2177474264,5545631253,14123715032,35970535581,91610417447,233315085507,594211124042,1513347751038,3854221711625,9816002298330
%N a(n+3) = 2*a(n+2) + a(n+1) + a(n) with a(0)=3, a(1)=2, a(2)=6.
%C Also the number of maximal independent vertex sets (and minimal vertex covers) on the 2n-crossed prism graph. - _Eric W. Weisstein_, Jun 15 2017
%C Also the number of irredundant sets in the n-sun graph. - _Eric W. Weisstein_, Aug 07 2017
%C Let {x,y,z} be the simple roots of P(x) = x^3 + u*x^2 + v*x + w. For n>=0, let p(n) = x^n/((x-y)(x-z)) + y^n/((y-x)(y-z)) + z^n/((z-x)(z-y)), q(n) = x^n + y^n + z^n. Then for n >= 0, q(n) = 3*p(n+2) + 2*u*p(n+1) + v*p(n). In this case, P(x) = x^3 - 2*x^2 - x - 1, q(n) = a(n), p(n) = A077939(n). - _Kai Wang_, Apr 15 2020
%C Also the number of tilings of a bracelet of length n with two colors of squares and one color of domino and tromino. - _Greg Dresden_ and Arnim Kuchhal, Aug 05 2024
%H Robert Israel, <a href="/A276225/b276225.txt">Table of n, a(n) for n = 0..2450</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/CrossedPrismGraph.html">Crossed Prism Graph</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/IrredundantSet.html">Irredundant Set</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/MaximalIndependentVertexSet.html">Maximal Independent Vertex Set</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/MinimalVertexCover.html">Minimal Vertex Cover</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/SunGraph.html">Sun Graph</a>
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,1).
%F Let p = (4*(61 + 9*sqrt(29)))^(1/3), q = (4*(61 - 9*sqrt(29)))^(1/3), and x = (1/6)*(4 + p + q) then x^n = (1/6)*(2*a(n) + A276226(n)*(p + q) + A077939(n-1)*(p^2 + q^2)).
%F G.f.: (3 - 4*x - x^2)/(1 - 2*x - x^2 - x^3).
%F a(n) = b^n + c^n + d^n, where (b, c, d) are the three roots of the cubic equation x^3 = 2*x^2 + x + 1.
%F a(n) = 3*A077939(n+2) - 4*A077939(n+1) - A077939(n). - _Kai Wang_, Apr 15 2020
%p f:= gfun:-rectoproc({a(n+3) = 2*a(n+2) + a(n+1) + a(n), a(0)=3, a(1)=2, a(2)=6},a(n),remember):
%p map(f, [$0..40]); # _Robert Israel_, Aug 29 2016
%t LinearRecurrence[{2, 1, 1}, {3, 2, 6}, 50]
%t CoefficientList[Series[(3 - 4 x - x^2)/(1 - 2 x - x^2 - x^3), {x, 0, 32}], x] (* _Michael De Vlieger_, Aug 25 2016 *)
%t Table[RootSum[-1 - #1 - 2 #1^2 + #1^3 &, #^n &], {n, 20}] (* _Eric W. Weisstein_, Jun 15 2017 *)
%o (Magma) I:=[3,2,6]; [n le 3 select I[n] else 2*Self(n-1)+ Self(n-2)+Self(n-3): n in [1..40]]; // _Vincenzo Librandi_, Aug 25 2016
%o (PARI) Vec((3-4*x-x^2)/(1-2*x-x^2-x^3) + O(x^99)) \\ _Altug Alkan_, Aug 25 2016
%Y Cf. A077939, A276226.
%K nonn,easy
%O 0,1
%A _G. C. Greubel_, Aug 24 2016