Search: id:A033570
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%I A033570
%S A033570 1,12,35,70,117,176,247,330,425,532,651,782,925,1080,1247,
%T A033570 1426,1617,1820,2035,2262,2501,2752,3015,3290,3577,3876,
%U A033570 4187,4510,4845,5192,5551,5922,6305,6700,7107,7526,7957
%N A033570 Pentagonal numbers with odd index: (2*n+1)*(3*n+1).
%C A033570 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
%F A033570 G.f.: A(x) = (1+9*x+2*x^2)/(1-x)^3.
%F A033570 a(n)=12*n+a(n-1)-13 (with a(1)=1) [From Vincenzo Librandi (vincenzo.librandi(AT)tin.it), 
               Nov 15 2009]
%e A033570 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]
%Y A033570 Cf. A000326, A049452, A033568.
%Y A033570 Sequence in context: A088596 A077293 A053682 this_sequence A163661 A142074 
               A102085
%Y A033570 Adjacent sequences: A033567 A033568 A033569 this_sequence A033571 A033572 
               A033573
%K A033570 nonn
%O A033570 0,2
%A A033570 N. J. A. Sloane (njas(AT)research.att.com).

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