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%I #30 Oct 13 2015 13:14:34
%S 1,3,9,12,30,18,45,27,66,33,81,42,102,48,117,57,138,63,153,72,174,78,
%T 189,87,210,93,225,102,246,108,261,117,282,123,297,132,318,138,333,
%U 147,354,153,369,162,390,168,405
%N Start with a single equilateral triangle for n=0; for the odd n-th generation add a triangle at each expandable vertex of the triangles of the (n-1)-th generation (this is the "side to vertex" version); for the even n-th generation use the "vertex to vertex" version; a(n) is the number of triangles added in the n-th generation.
%C See a comment on V-V and V-S at A249246.
%C The overlap rules for the expansion are: (i) overlap within generation is allowed. (ii) overlap of different generations is prohibited.
%C There are a total of 16 combinations as shown in the table below:
%C +-------------------------------------------------------+
%C | Even n-th version V-V S-V V-S S-S |
%C +-------------------------------------------------------+
%C | Odd n-th version |
%C | V-V A008486 A248969 A261951 A261952 |
%C | S-V a(n) A008486 A008486 A261956 |
%C | V-S A249246 A008486 A008486 A261957 |
%C | S-S A261953 A261954 A261955 A008486 |
%C +-------------------------------------------------------+
%C Note: V-V = vertex to vertex, S-V = side to vertex,
%C V-S = vertex to side, S-S = side to side.
%H Kival Ngaokrajang, <a href="/A261950/a261950_2.pdf">Illustration of initial terms</a>
%F Conjectures from _Colin Barker_, Sep 10 2015: (Start)
%F a(n) = a(n-2)+a(n-4)-a(n-6) for n>6.
%F G.f.: (7*x^6+3*x^5+20*x^4+9*x^3+8*x^2+3*x+1) / ((x-1)^2*(x+1)^2*(x^2+1)).
%F (End)
%o (PARI) {e=9; o=3; print1("1, ", o, ", ", e, ", "); for(n=3, 100, if (Mod(n,2)==0, if (Mod(n,4)==0, e=e+21); if (Mod(n,4)==2, e=e+15); print1(e, ", "), if (Mod(n,4)==3, o=o+9); if (Mod(n,4)==1, o=o+6); print1(o, ", ")))}
%Y Cf. A008486, A248969, A249246.
%K nonn
%O 0,2
%A _Kival Ngaokrajang_, Sep 06 2015
%E Typo in data fixed by _Colin Barker_, Sep 10 2015
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