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Search: id:A033570
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| A033570 |
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Pentagonal numbers with odd index: (2*n+1)*(3*n+1). |
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+0
7
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| 1, 12, 35, 70, 117, 176, 247, 330, 425, 532, 651, 782, 925, 1080, 1247, 1426, 1617, 1820, 2035, 2262, 2501, 2752, 3015, 3290, 3577, 3876, 4187, 4510, 4845, 5192, 5551, 5922, 6305, 6700, 7107, 7526, 7957
(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 3-subset of an 2n-set X then, for n>=4, a(n-2) is the number of 4-subsets of X having at least two elements in common with Y. - Milan R. Janjic (agnus(AT)blic.net), Dec 16 2007
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FORMULA
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G.f.: A(x) = (1+9*x+2*x^2)/(1-x)^3.
a(n)=12*n+a(n-1)-13 (with a(1)=1) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 15 2009]
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EXAMPLE
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For n=2, a(2)=12*2+1-13=12; n=3, a(3)=12*3+12-13=35; n=4, a(4)=12*4+35-13=70 [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), Nov 15 2009]
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CROSSREFS
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Cf. A000326, A049452, A033568.
Sequence in context: A088596 A077293 A053682 this_sequence A163661 A142074 A102085
Adjacent sequences: A033567 A033568 A033569 this_sequence A033571 A033572 A033573
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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