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Search: id:A140661
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| A140661 |
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Number of pairs (b,c) with the same prime factors, 1<=b<=c<=n. |
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+0
1
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| 2, 3, 5, 6, 7, 8, 11, 13, 14, 15, 17, 18, 19, 20, 24, 25, 28, 29, 31, 32, 33, 34, 38, 40, 41, 44, 46, 47, 48, 49, 54, 55, 56, 57, 62, 63, 64, 65, 68, 69, 70, 71, 73, 75, 76, 77, 83, 85, 89, 90, 92, 93, 100, 101, 104, 105, 106, 107, 109, 110, 111, 113, 119, 120, 121, 122
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OFFSET
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2,1
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COMMENT
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If pairs are restricted to b<c, we get the variant 0, 0, 1, 1, 1, 1, 3, 4, 4, 4, 5, 5, 5, 5, 8, 8, 10, 10, 11, 11, 11, 11, 14, 15,...
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LINKS
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P. Erdos and T. Motzkin, Density of pairs with same prime factors, Am. Math. Month. vol 97 no 10 (1990) p 937, problem 5735.
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EXAMPLE
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a(16)=24 counts the 16 pairs (b,b) with 1<=b<=16 plus the 8 pairs (2,4), (2,8), (2,16), (4,8), (4,16), (8,16), (3,9), (6,12).
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CROSSREFS
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Adjacent sequences: A140658 A140659 A140660 this_sequence A140662 A140663 A140664
Sequence in context: A028778 A094180 A116545 this_sequence A063966 A123030 A063752
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KEYWORD
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easy,nonn
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AUTHOR
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R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jul 11 2008
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