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Search: id:A140128
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| A140128 |
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A positive integer n is included if d(d(n)) = d(d(n+1)), where d(n) is the number of divisors of n. |
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1
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| 2, 3, 4, 14, 16, 21, 26, 33, 34, 35, 38, 44, 57, 75, 85, 86, 93, 94, 98, 104, 116, 118, 122, 133, 135, 141, 142, 145, 147, 152, 153, 158, 164, 170, 171, 174, 175, 177, 188, 189, 201, 202, 205, 207, 213, 214, 217, 218, 225, 230, 231, 242, 243, 244, 245
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OFFSET
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1,1
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FORMULA
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For m = 2,3,4,5..., a(m) is the smallest integer > a(m-1) such that A010553(a(m)) = A010553(a(m)+1).
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EXAMPLE
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35 has 4 divisors, and 4 has 3 divisors. 36 has 9 divisors, and 9 has 3 divisors. Since d(d(35)) = d(d(36)) (=3), then 35 is included in the sequence.
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MATHEMATICA
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Select[Range[250], DivisorSigma[0, DivisorSigma[0, # ]] == DivisorSigma[0, DivisorSigma[0, # + 1]] &] - Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Jun 05 2008
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CROSSREFS
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Cf. A010553.
Sequence in context: A039576 A091907 A103048 this_sequence A100998 A127283 A047193
Adjacent sequences: A140125 A140126 A140127 this_sequence A140129 A140130 A140131
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KEYWORD
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nonn
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AUTHOR
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Leroy Quet (q1qq2qqq3qqqq(AT)yahoo.com), Jun 04 2008
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EXTENSIONS
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More terms from Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Jun 05 2008
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