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Search: id:A124080
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| A124080 |
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10 times triangular numbers. |
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+0
7
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| 0, 10, 30, 60, 100, 150, 210, 280, 360, 450, 550, 660, 780, 910, 1050, 1200, 1360, 1530, 1710, 1900, 2100, 2310, 2530, 2760, 3000, 3250, 3510, 3780, 4060, 4350, 4650, 4960, 5280, 5610, 5950, 6300, 6660, 7030, 7410, 7800, 8200, 8610, 9030, 9460, 9900, 10350
(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 is a 5-subset of an n-set X then, for n>=5, a(n-4) is equal to the number of 5-subsets of X having exactly three elements in common with Y. Y is a 5-subset of an n-set X then, for n>=6, a(n-6) is the number of (n-5)-subsets of X having exactly two elements in common with Y.lso, if - Milan R. Janjic (agnus(AT)blic.net), Dec 28 2007
Except for the first term, a(n)=10*n+a(n-1), (with a(1)=10) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Oct 24 2009]
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FORMULA
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a(n)=10*C(n,2), n>=1
a(n)=A049598-A002378. - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 06 2007
a(n)=n*(n+1)*5, n>=0 . - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 06 2007
a(n) = 5n^2 + 5n = A000217(n)*10 = A002378(n)*5 = A028895(n)*2. [From Omar E. Pol (info(AT)polprimos.com), Dec 12 2008]
a(n)=10*n+a(n-1)-10 (with a(1)=0) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 12 2009]
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EXAMPLE
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For n=2, a(2)=10*2-0-10=10; n=3, a(3)=10*3+10-10=30; n=4, a(4)=10*4+30-10=60 [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 12 2009]
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MAPLE
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[seq(10*binomial(n, 2), n=1..51)];
seq(n*(n+1)*5, n=0..39); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 06 2007
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MATHEMATICA
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s=0; lst={s}; Do[s+=n++ +10; AppendTo[lst, s], {n, 0, 8!, 10}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Nov 17 2008]
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CROSSREFS
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Cf. A028895, A046092, A045943, A002378, A028896, A024966, A033996, A027468.
Cf. A002378, A049598.
Cf. A000217. [From Omar E. Pol (info(AT)polprimos.com), Dec 12 2008]
Sequence in context: A096844 A031299 A104044 this_sequence A034127 A005052 A057344
Adjacent sequences: A124077 A124078 A124079 this_sequence A124081 A124082 A124083
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KEYWORD
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easy,nonn,new
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AUTHOR
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Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Nov 24 2006
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