Search: id:A102366
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%I A102366
%S A102366 1,1,2,3,6,10,15,21,28,84,120,165,220,286,364,455,1820,2380,3060,3876,
%T A102366 4845,5985,7315,8855,10626,53130,65780,80730,98280,118755,142506,169911,
%U A102366 201376,237336,278256,324632,1947792,2324784,2760681,3262623,3838380
%N A102366 Number of subsets of {1,2,...,n} in which exactly half of the elements 
               are less than or equal to sqrt(n).
%F A102366 a(n) = Sum_k C(floor[sqrt(n)], k)*C(n-floor[sqrt(n)], k) =A048093(n)+1 
               =a(n-1)+A084919(n-1)
%e A102366 a(5)=10 since the ten subsets of {1,2,3,4,5} are { }, {1, 3}, {1, 4}, 
               {1, 5}, {2, 3}, {2, 4}, {2, 5}, {1,2, 3,4}, {1,2, 3,5} and {1,2, 
               4,5}.
%Y A102366 Cf. A011782 for number of subsets with an even number of elements.
%Y A102366 Sequence in context: A084396 A090035 A111467 this_sequence A152452 A074134 
               A056178
%Y A102366 Adjacent sequences: A102363 A102364 A102365 this_sequence A102367 A102368 
               A102369
%K A102366 nonn
%O A102366 0,3
%A A102366 Henry Bottomley (se16(AT)btinternet.com), Feb 22 2005

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