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A321257
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Start with an equilateral triangle, and repeatedly append along the triangles of the previous step equilateral triangles with half their side length that do not overlap with any prior triangle; a(n) gives the number of triangles appended at n-th step.
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3
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1, 6, 21, 60, 147, 330, 705, 1464, 2991, 6054, 12189, 24468, 49035, 98178, 196473, 393072, 786279, 1572702, 3145557, 6291276, 12582723, 25165626, 50331441, 100663080, 201326367, 402652950, 805306125, 1610612484, 3221225211, 6442450674, 12884901609, 25769803488
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OFFSET
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1,2
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COMMENTS
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The following diagram depicts the first three steps of the construction:
- - - -
/ \ / \ / \ / \
/ 3 \ / 3 \ / 3 \ / 3 \
-------------------------
/ \ / \ / \
/ 3 \ 2 / \ 2 / 3 \
------- / \ -------
/ \ / \ / \ / \ / \
/ 3 \ / 3 \ / \ / 3 \ / 3 \
------------- -------------
/ \ / \ / \
/ 3 \ 2 / 1 \ 2 / 3 \
------- / \ -------
/ \ / \ / \
/ 3 \ / \ / 3 \
-------------------------------------
/ \ / \ / \
/ 3 \ 2 / 3 \ 2 / 3 \
------- ------- -------
/ \ / \ / \ / \
/ 3 \ / 3 \ / 3 \ / 3 \
-------------------------
A triangle of step n+1 touches one or two triangles of step n.
The construction presents holes from the 3rd step onwards; these will be gradually filled in the subsequent steps.
The limiting construction is a hexagon; its area is 6 times the area of the initial triangle.
See A321237 for a similar sequence.
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LINKS
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FORMULA
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a(n) = 3*(2^(n-1) + 3*(2^(n-1)-n)) for any n > 1.
Sum_{n > 0} a(n) / 4^(n-1) = 6.
G.f.: x*(1 + 2*x + 2*x^2 + 4*x^3)/((1-2*x)*(1-x)^2). - Vincenzo Librandi, Nov 02 2018
a(n) - 4*a(n-1) + 5*a(n-2) - 2*a(n-3) = 0, with n>1. - Vincenzo Librandi, Nov 02 2018
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MAPLE
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1, seq(3*(2^(n-1)+3*(2^(n-1)-n)), n=2..35); # Muniru A Asiru, Nov 02 2018
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MATHEMATICA
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CoefficientList[Series[(1 + 2 x + 2 x^2 + 4 x^3) / ((1 - 2 x) (1 - x)^2), {x, 0, 40}], x] (* Vincenzo Librandi, Nov 02 2018 *)
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PROG
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(PARI) a(n) = if (n==1, 1, 3*(2^(n-1) + 3*(2^(n-1)-n)))
(Magma) [1] cat [3*(2^(n-1) + 3*(2^(n-1)-n)): n in [2..35]]; // Vincenzo Librandi, Nov 02 2018
(GAP) Concatenation([1], List([2..35], n->3*(2^(n-1)+3*(2^(n-1)-n)))); # Muniru A Asiru, Nov 02 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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