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%I #7 Apr 02 2019 19:19:13
%S 0,1,4,18,82,377,1740,8045,37226,172314,797744,3693493,17101128,
%T 79180525,366618808,1697509962,7859781454,36392245541,168502887396,
%U 780199897985,3612471696230,16726421117538,77446465948772,358591660029577,1660346632032144,7687716275234809,35595568065121900,164814155562334914
%N Self-composition of the Pell numbers; g.f.: A(x) = G(G(x)), where G(x) = g.f. of A000129.
%H N. J. A. Sloane, <a href="/transforms.txt">Transforms</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PellNumber.html">Pell Number</a>
%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (6,-5,-6,-1).
%F G.f.: x*(1 - 2*x - x^2)/(1 - 6*x + 5*x^2 + 6*x^3 + x^4).
%F a(n) = 6*a(n-1) - 5*a(n-2) - 6*a(n-3) - a(n-4).
%t CoefficientList[Series[x (1 - 2 x - x^2)/(1 - 6 x + 5 x^2 + 6 x^3 + x^4), {x, 0, 27}], x]
%t LinearRecurrence[{6, -5, -6, -1}, {0, 1, 4, 18}, 28]
%Y Cf. A000129, A270863.
%K nonn,easy
%O 0,3
%A _Ilya Gutkovskiy_, Dec 09 2016