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Search: id:A028896
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| A028896 |
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6 times triangular numbers. |
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+0
13
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| 0, 6, 18, 36, 60, 90, 126, 168, 216, 270, 330, 396, 468, 546, 630, 720, 816, 918, 1026, 1140, 1260, 1386, 1518, 1656, 1800, 1950, 2106, 2268, 2436, 2610, 2790, 2976, 3168, 3366, 3570, 3780, 3996, 4218, 4446, 4680, 4920, 5166, 5418, 5676
(list; graph; listen)
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OFFSET
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0,2
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COMMENT
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Write 1,2,3,4,... in a hexagonal spiral around 0, then a(n) is the sequence found by reading the line from 0 in the direction 0,6,... - Floor van Lamoen (fvlamoen(AT)hotmail.com), Jul 21 2001. The spiral begins:
......16..15..14
....17..5...4...13
..18..6...0...3...12
19..7...1...2...11..26
..20..8...9...10..25
....21..22..23..24
If Y is a 4-subset of an n-set X then, for n>=5, a(n-5) is the number of (n-4)-subsets of X having exactly two elements in common with Y. - Milan R. Janjic (agnus(AT)blic.net), Dec 28 2007
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LINKS
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Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
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FORMULA
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a(n) = 3*n*(n+1). G.f.: A(x) = 6*x/(1-x)^3.
Polygorial(3, n+1) - Daniel Dockery (peritus(AT)gmail.com) Jun 16, 2003
a(n)=A049598/2; a(n)=A124080-A046092; a(n)=A033996-A002378. - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 06 2007
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MAPLE
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[seq(6*binomial(n, 2), n=1..44)]; - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Nov 24 2006
a:=n->sum(sum(3, j=1..n), k=0..n): seq(a(n), n=0..43); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), May 30 2007
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CROSSREFS
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Cf. A000567.
Cf. A003215, A028895, A024966.
Cf. A084939, A084940, A084941, A084942, A084943, A084944.
Cf. A028895, A046092, A045943, A002378.
Cf. A049598, A124080, A046092, A033996, A002378.
Sequence in context: A110965 A111147 A069958 this_sequence A034857 A116367 A101853
Adjacent sequences: A028893 A028894 A028895 this_sequence A028897 A028898 A028899
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KEYWORD
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nonn,easy
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AUTHOR
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Joe Keane (jgk(AT)jgk.org)
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