|
|
A248969
|
|
Start with a single equilateral triangle; at odd n-th generation add a similar triangle at each expandable vertex (this is the "vertex to vertex" version); alternate with the "side to vertex" version for even n-th generation; a(n) is the number of triangle for each generation.
|
|
10
|
|
|
1, 3, 6, 15, 18, 42, 24, 57, 30, 72, 36, 87, 48, 114, 54, 129, 60, 144, 66, 159, 78, 186, 84, 201, 90, 216, 96, 231, 108, 258, 114, 273, 120, 288, 126, 303, 138, 330, 144, 345, 150, 360, 156, 375, 168, 402, 174, 417, 180, 432, 186, 447, 198, 474, 204, 489, 210, 504, 216, 519
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,2
|
|
COMMENTS
|
The construction rules alternate between "vertex to vertex" (A061777 & companions) and "side to vertex" (A101946 & companions). See the link for an illustration.
|
|
LINKS
|
|
|
FORMULA
|
Empirical g.f.: (3*x^11 +x^10 +12*x^9 +5*x^8 +15*x^7 +6*x^6 +27*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 18 2014
|
|
PROG
|
(PARI)
{
c2=0; c3=0; c6=3; c7=1; c8=0;
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
c4=(n^2-1)/16;
if (c4==floor(c4), c5=-1, c5=1);
if (n>4, c6=c6+c5);
if (n>=2, c7=c7+c6);
if (c6<>4, c9=0, c9=2);
a=3*(c7+c8+c9);
c8=c7
);
print1(a", ")
)
}
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|