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Search: id:A046691
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| -1, 2, 6, 11, 17, 24, 32, 41, 51, 62, 74, 87, 101, 116, 132, 149, 167, 186, 206, 227, 249, 272, 296, 321, 347, 374, 402, 431, 461, 492, 524, 557, 591, 626, 662, 699, 737, 776, 816, 857, 899, 942, 986, 1031, 1077, 1124, 1172, 1221, 1271, 1322
(list; graph; listen)
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OFFSET
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0,2
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COMMENT
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If Y_i (i=1,2,3,4) are 2-blocks of an n-set X then, for n>=8, a(n-3) is the number of (n-2)-subsets of X intersecting each Y_i (i=1,2,3,4). - Milan R. Janjic (agnus(AT)blic.net), Nov 09 2007
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LINKS
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Milan Janjic, Two Enumerative Functions
P. Di Francesco, O. Golinelli and E. Guitter, Meander, folding and arch statistics.
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FORMULA
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G.f.: A(x) = (-1+5*x-3*x^2)/(1-x)^3.
a(n)=n+a(n-1)+1 (with a(1)=-1) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 18 2009]
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EXAMPLE
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For n=2, a(2)=2-1+1=2; n=3, a(3)=3+2+1=6; n=4, a(4)=4+6+1=11 [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 18 2009]
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MAPLE
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seq(binomial(n, 2)-4, n=3..52); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jan 13 2007
with (combinat):seq((fibonacci(3, n)+n-9)/2, n=2..51); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 07 2008
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MATHEMATICA
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i=3; s=-1; lst={}; Do[s+=n+i; If[s>=0, AppendTo[lst, s]], {n, 0, 6!, 1}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Oct 29 2008]
s=-1; lst={}; Do[s+=n-2; If[s>-2, AppendTo[lst, s]], {n, 0, 6!, 1}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Nov 04 2008]
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CROSSREFS
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Triangular numbers (A000217) minus 4. Cf. A027379.
Cf. A002522.
Sequence in context: A020966 A078476 A099056 this_sequence A098167 A081689 A104813
Adjacent sequences: A046688 A046689 A046690 this_sequence A046692 A046693 A046694
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KEYWORD
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sign,new
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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