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 A249246 Start with a single equilateral triangle for n=0; for the odd n-th generation add a triangle at each expandable side of the triangles of the (n-1)-th generation (this is the "vertex to side" version); for the even n-th generation use the "vertex to vertex" version; a(n) is the number of triangles in the n-th generation. 10
 1, 3, 6, 15, 18, 30, 24, 45, 30, 60, 36, 75, 48, 90, 54, 105, 60, 120, 66, 135, 78, 150, 84, 165, 90, 180, 96, 195, 108, 210, 114, 225, 120, 240, 126, 255, 138, 270, 144, 285, 150, 300, 156, 315, 168, 330, 174, 345, 180, 360, 186, 375, 198, 390, 204, 405, 210, 420, 216, 435 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The construction rules alternate between "vertex to side" (A101946 & companions) and "vertex to vertex" (A061777 & companions). 'Vertex to side' means vertex of n-th generation triangle touches the middle of a side of the (n-1)-th generation triangle. See the link with an illustration. The even terms are the same as in A248969. Note that the triangles overlap. LINKS Kival Ngaokrajang, Illustration of initial terms FORMULA Empirical g.f.: (3*x^11 + x^10 + 12*x^9 + 5*x^8 + 15*x^7 + 6*x^6 + 15*x^5 + 12*x^4 + 12*x^3 + 5*x^2 + 3*x + 1) / ((x-1)^2*(x+1)^2*(x^2+1)*(x^4+1)). - Colin Barker, Oct 24 2014 PROG (PARI) { c2=0; c3=0; c5=3; for(n=0, 100,    if (Mod(n, 2)==0,    \\even        if (n<1, a(n)=1, c3=c3+c2; a=6*c3);        c1=n/8+3/4;        if (c1==floor(c1), c2=2, c2=1)    ,    \\odd        a=c5;        if (n<=1, c4=12, c4=15);        c5=c5+c4    );    print1(a", ") ) } CROSSREFS Vertex to vertex: A061777, A247618, A247619, A247620. Vertex to side: A101946, A247903, A247904, A247905. Cf. A248969. Sequence in context: A124518 A160724 A212060 * A248969 A174279 A233554 Adjacent sequences:  A249243 A249244 A249245 * A249247 A249248 A249249 KEYWORD nonn AUTHOR Kival Ngaokrajang, Oct 23 2014 EXTENSIONS Edited. Name and comment reformulated. - Wolfdieter Lang, Nov 04 2014 STATUS approved

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Last modified June 20 04:43 EDT 2021. Contains 345157 sequences. (Running on oeis4.)