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A060423
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Number of obtuse triangles made from vertices of a regular n-gon.
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1
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0, 0, 0, 0, 0, 5, 6, 21, 24, 54, 60, 110, 120, 195, 210, 315, 336, 476, 504, 684, 720, 945, 990, 1265, 1320, 1650, 1716, 2106, 2184, 2639, 2730, 3255, 3360, 3960, 4080, 4760, 4896, 5661, 5814, 6669, 6840, 7790, 7980, 9030, 9240, 10395, 10626
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,6
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FORMULA
| n(n-1)(n-3)/8 when n odd; n(n-2)(n-4)/8 when n even
G.f.: x^5(x+5)/((1-x)(1-x^2)^3). - Michael Somos, Jan 30 2004
For n odd, a(n) = A080838(n). [From Gerald McGarvey (gerald.mcgarvey(AT)comcast.net), Sep 14 2008]
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PROG
| (PARI) a(n)=polcoeff(x^5*(5+x)/(1-x)/(1-x^2)^3+x*O(x^n), n)
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CROSSREFS
| Cf. A007290, A046092, A000330.
Sequence in context: A133608 A072577 A057520 * A037951 A095308 A132796
Adjacent sequences: A060420 A060421 A060422 * A060424 A060425 A060426
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KEYWORD
| easy,nice,nonn
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AUTHOR
| Sen-Peng You (giawgwan(AT)single.url.com.tw), Apr 05 2001
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