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Search: id:A089822
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| A089822 |
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Number of subsets of {1,.., n} containing exactly two primes. |
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+0
5
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| 0, 0, 2, 4, 12, 24, 48, 96, 192, 384, 640, 1280, 1920, 3840, 7680, 15360, 21504, 43008, 57344, 114688, 229376, 458752, 589824, 1179648, 2359296, 4718592, 9437184, 18874368, 23592960, 47185920, 57671680, 115343360, 230686720
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OFFSET
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1,3
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COMMENT
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a(n) = A000217(A000720(n)-1)*A089819(n);
for n>2: a(n) = A089818(n,2).
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FORMULA
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a(n) = (pi(n)*(pi(n)-1)*2^(n-pi(n)))/2, with pi = A000720.
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EXAMPLE
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a(5)=12 subsets of {1,2,3,4,5} contain exactly two primes: {2,3},
{2,5}, {3,5}, {1,2,3}, {1,2,5}, {1,3,5}, {2,3,4}, {2,4,5}, {3,4,5},
{1,2,3,4}, {1,2,4,5} and {1,3,4,5}.
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CROSSREFS
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Cf. A089821.
Sequence in context: A059322 A163908 A108720 this_sequence A079352 A089888 A133411
Adjacent sequences: A089819 A089820 A089821 this_sequence A089823 A089824 A089825
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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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