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A026551 a(n) = Sum{T(i,j)}, 0<=j<=i, 0<=i<=2n, T given by A026536. 4

%I

%S 3,9,21,57,129,345,777,2073,4665,12441,27993,74649,167961,447897,

%T 1007769,2687385,6046617,16124313,36279705,96745881,217678233,

%U 580475289,1306069401,3482851737,7836416409,20897110425,47018498457

%N a(n) = Sum{T(i,j)}, 0<=j<=i, 0<=i<=2n, T given by A026536.

%C The even terms are the number of holes of SierpiƄski triangle-like fractals. The odd terms are the total number of holes and triangles. - _Kival Ngaokrajang_, Mar 30 2014

%C All terms are divisible by 3 (see g.f.). - _Joerg Arndt_, Dec 20 2014

%H Kival Ngaokrajang, <a href="/A026551/a026551.pdf">Illustration of initial terms</a>

%F G.f.: 3*(1+2*x-2*x^2)/((1-x)*(1-6*x^2)). - _Ralf Stephan_, Feb 03 2004

%o (PARI) Vec( 3*(1+2*x-2*x^2)/((1-x)*(1-6*x^2))+O(x^33)); \\ _Joerg Arndt_, Dec 20 2014

%Y Cf. A026534, A026565.

%K nonn,easy

%O 0,1

%A _Clark Kimberling_

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Last modified September 18 16:22 EDT 2020. Contains 337170 sequences. (Running on oeis4.)