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Search: id:A089821
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| A089821 |
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Number of subsets of {1,.., n} containing exactly one prime. |
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+0
4
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| 0, 2, 4, 8, 12, 24, 32, 64, 128, 256, 320, 640, 768, 1536, 3072, 6144, 7168, 14336, 16384, 32768, 65536, 131072, 147456, 294912, 589824, 1179648, 2359296, 4718592, 5242880, 10485760, 11534336, 23068672, 46137344, 92274688
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OFFSET
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1,2
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COMMENT
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a(n )= A000720(n)*A089819(n);
for n>1: a(n) = A089818(n,1).
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FORMULA
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a(n) = pi(n) * 2^(n-pi(n)), with pi = A000720.
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EXAMPLE
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a(5)=12 subsets of {1,2,3,4,5} contain exactly one prime: {2},
{3}, {5}, {1,2}, {1,3}, {1,5}, {2,4}, {3,4}, {4,5}, {1,2,4}, {1,3,4} and {1,4,5}.
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CROSSREFS
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Cf. A089822.
Sequence in context: A103787 A032473 A084422 this_sequence A097942 A004653 A115386
Adjacent sequences: A089818 A089819 A089820 this_sequence A089822 A089823 A089824
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KEYWORD
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nonn
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AUTHOR
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Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Nov 12 2003
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