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A261957
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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 "side to side" version; a(n) is the number of triangles added in the n-th generation.
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8
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1, 3, 9, 12, 24, 12, 24, 18, 36, 33, 57, 45, 81, 36, 78, 42, 90, 57, 111, 69, 135, 60, 132, 66, 144, 81, 165, 93, 189, 84, 186, 90, 198, 105, 219, 117, 243, 108, 240, 114, 252, 129, 273, 141, 297, 132, 294, 138, 306, 153
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OFFSET
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0,2
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COMMENTS
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See a comment on V-V and V=S at A249246.
The overlap rules for the expansion are: (i) overlap within generation is allowed. (ii) overlap of different generations is prohibited.
There are a total of 16 combinations as shown in the table below:
+-------------------------------------------------------+
| Even n-th version V-V S-V V-S S-S |
+-------------------------------------------------------+
| Odd n-th version |
+-------------------------------------------------------+
Note: V-V = vertex to vertex, S-V = side to vertex,
V-S = vertex to side, S-S = side to side.
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LINKS
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FORMULA
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a(n) = a(n-2)+a(n-8)-a(n-10) for n>14.
G.f.: (3*x^14+9*x^13-9*x^12-3*x^11-13*x^10-12*x^9-11*x^8-6*x^7-15*x^4-9*x^3-8*x^2-3*x-1) / ((x-1)^2*(x+1)^2*(x^2+1)*(x^4+1)).
(End)
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PROG
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(PARI) {e=24; o=12; print1("1, 3, 9, 12, 24, ", o, ", ", e, ", "); for(n=7, 100, if (Mod(n, 2)==0, if (Mod(n, 8)==0, e=e+12); if (Mod(n, 8)==2, e=e+21); if (Mod(n, 8)==4, e=e+24); if (Mod(n, 8)==6, e=e-3); print1(e, ", "), if (Mod(n, 8)==7, o=o+6); if (Mod(n, 8)==1, o=o+15); if (Mod(n, 8)==3, o=o+12); if (Mod(n, 8)==5, o=o-9); print1(o, ", ")))}
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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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