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A343485
Area of the convex hull around terdragon expansion level n, measured in unit triangles.
2
0, 2, 8, 26, 86, 276, 856, 2586, 7826, 23628, 71128, 213546, 641246, 1925076, 5777416, 17333706, 52006586, 156031788, 468115048, 1404358266, 4213124006, 12639480276, 37918617976, 113755972026, 341268358946, 1023806051148, 3071419747768, 9214260306186
OFFSET
0,2
COMMENTS
Expansion level n comprises the first 3^n segments of the curve.
LINKS
FORMULA
For n>=2, a(n) = (29/24)*3^n - (h/12)*3^floor(n/2) - (c/8) where h = 15,23,11,25 and c = 5,3,1,3 according as n == 0,1,2,3 (mod 4) respectively.
a(n) = 4*a(n-1) - 4*a(n-2) + 4*a(n-3) + 6*a(n-4) - 36*a(n-5) + 36*a(n-6) - 36*a(n-7) + 27*a(n-8), for n>=10.
G.f.: (2*x + 2*x^3 + 6*x^4 - 8*x^5 + 16*x^6 - 18*x^7 + 6*x^8 - 18*x^9) /( (1-x)*(1+x^2)*(1-9*x^4)*(1-3*x) ).
G.f.: (1/24)*( 16 + 16*x - 9/(1-x) - 6/(1+x^2) - (26+48*x)/(1-3*x^2) + (-4+2*x)/(1+3*x^2) + 29/(1-3*x) ).
Lim_{n->oo} a(n)/3^n = 29/24.
EXAMPLE
For n=1, the terdragon curve comprises 3 segments:
@---@ Convex hull vertices are marked "@".
\ They enclose an area of 2 unit triangles,
@---@ so a(1) = 2.
.
For n=2, the terdragon curve comprises 9 segments:
@---@
\ Convex hull vertices are marked "@".
@---* They enclose an area of a(2) = 8
\ / \ unit triangle equivalents.
*---@
\
@---@
PROG
(PARI) my(h=[30, 46, 22, 50]); a(n) = if(n<2, 2*n, (29*3^n - h[n%4+1]*3^(n\2))\24);
CROSSREFS
Cf. A343486 (fractal hull area), A341029 (dragon curve hull area).
Sequence in context: A128634 A230904 A289595 * A298189 A053956 A320802
KEYWORD
nonn,easy
AUTHOR
Kevin Ryde, Apr 17 2021
STATUS
approved