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Search: id:A051938
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| A051938 |
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Truncated triangular numbers: a(n)=n*(n+1)/2-18. |
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+0
1
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| 3, 10, 18, 27, 37, 48, 60, 73, 87, 102, 118, 135, 153, 172, 192, 213, 235, 258, 282, 307, 333, 360, 388, 417, 447, 478, 510, 543, 577, 612, 648, 685, 723, 762, 802, 843, 885, 928, 972, 1017, 1063, 1110, 1158, 1207, 1257, 1308, 1360, 1413, 1467, 1522, 1578
(list; graph; listen)
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OFFSET
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6,1
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COMMENT
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If a 3-set Y and a 3-set Z, having one element in common, are subsets of an n-set X then a(n+2) is the number of 3-subests of X intersecting both Y and Z. - Milan R. Janjic (agnus(AT)blic.net), Oct 03 2007
Numbers n such that a(n)=2*a(n-1)-a(n-2)+1, with a(1)=3, a(2)=10. example: For n=3, a(3)=2*a(2)-a(1)+1=2*10-3+1=18; n=4, a(4)=27 [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Jan 22 2009]
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LINKS
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Milan Janjic, Two Enumerative Functions
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FORMULA
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a(n)=n+a(n-1)+5 (with a(1)=3) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 19 2009]
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EXAMPLE
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For n=2, a(2)=2+3+5=10; n=3, a(3)=3+10+5=18; n=4, a(4)=4+18+5=27 [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 19 2009]
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MATHEMATICA
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i=6; s=-3; lst={}; Do[s+=n+i; AppendTo[lst, s], {n, 0, 6!, 1}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Oct 30 2008]
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CROSSREFS
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a(n)=A000217(n)-18, n>5
Cf. A155212 [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Jan 22 2009]
Sequence in context: A063211 A063111 A031063 this_sequence A074893 A074178 A127852
Adjacent sequences: A051935 A051936 A051937 this_sequence A051939 A051940 A051941
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KEYWORD
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easy,nice,nonn,new
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AUTHOR
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Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 21 1999
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