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%I #8 Sep 08 2022 08:45:25
%S 1,1,2,4,11,37,164,1000,8021,81001,1076006,19683244,473632031,
%T 14349084877,571833704648,31381448626000,2265367321680041,
%U 205893684435186001,24615565942378859210,4052605390737766057684
%N Antidiagonal sums of triangle A118180: a(n) = Sum_{k=0..[n/2]} (3^k)^(n-2*k) for n>=0.
%H G. C. Greubel, <a href="/A118182/b118182.txt">Table of n, a(n) for n = 0..125</a>
%F G.f.: A(x) = Sum_{n>=0} x^n/(1-3^n*x^2).
%F a(2*n) = Sum_{k=0..n} (3^k)^(2*(n-k)).
%F a(2*n+1) = Sum_{k=0..n} (3^k)^(2*(n-k) +1).
%e A(x) = 1/(1-x^2) + x/(1-3x^2) + x^2/(1-9x^2) + x^3/(1-27x^2) +...
%e = 1 + x + 2*x^2 + 4*x^3 + 11*x^4 + 37*x^5 + 164*x^6 + 1000*x^7 +...
%t Table[Sum[3^(k*(n-2*k)), {k,0,Floor[n/2]}], {n,0,30}] (* _G. C. Greubel_, Jun 29 2021 *)
%o (PARI) a(n)=sum(k=0, n\2, (3^k)^(n-2*k) );
%o (Magma) [(&+[3^(k*(n-2*k)): k in [0..Floor(n/2)]]): n in [0..30]]; // _G. C. Greubel_, Jun 29 2021
%o (Sage) [sum(3^(k*(n-2*k)) for k in (0..n//2)) for n in (0..30)] # _G. C. Greubel_, Jun 29 2021
%Y Cf. A118180 (triangle), A118181 (row sums), A118183, A118184.
%K nonn
%O 0,3
%A _Paul D. Hanna_, Apr 15 2006
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