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A026551 a(n) = Sum{T(i,j)}, 0<=j<=i, 0<=i<=2n, T given by A026536. 4
3, 9, 21, 57, 129, 345, 777, 2073, 4665, 12441, 27993, 74649, 167961, 447897, 1007769, 2687385, 6046617, 16124313, 36279705, 96745881, 217678233, 580475289, 1306069401, 3482851737, 7836416409, 20897110425, 47018498457 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

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

All terms are divisible by 3 (see g.f.). - Joerg Arndt, Dec 20 2014

LINKS

Table of n, a(n) for n=0..26.

Kival Ngaokrajang, Illustration of initial terms

FORMULA

G.f.: 3*(1+2*x-2*x^2)/((1-x)*(1-6*x^2)). - Ralf Stephan, Feb 03 2004

PROG

(PARI) Vec( 3*(1+2*x-2*x^2)/((1-x)*(1-6*x^2))+O(x^33)); \\ Joerg Arndt, Dec 20 2014

CROSSREFS

Cf. A026534, A026565.

Sequence in context: A272265 A191796 A007056 * A296719 A060578 A147078

Adjacent sequences:  A026548 A026549 A026550 * A026552 A026553 A026554

KEYWORD

nonn,easy

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified August 10 16:56 EDT 2020. Contains 336381 sequences. (Running on oeis4.)