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A036436
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Numbers whose number of divisors is a square.
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6
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1, 6, 8, 10, 14, 15, 21, 22, 26, 27, 33, 34, 35, 36, 38, 39, 46, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, 95, 100, 106, 111, 115, 118, 119, 120, 122, 123, 125, 129, 133, 134, 141, 142, 143, 145, 146, 155, 158, 159, 161, 166, 168, 177, 178, 183
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OFFSET
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1,2
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COMMENTS
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Invented by the HR (named after Hardy and Ramanujan) concept formation program.
Numbers in this sequence but not in A036455 are 1, 1260, 1440, 1800, 1980 etc. [From R. J. Mathar, Oct 20 2008]
tau(p^(n^2-1)) = n^2 so numbers of this form are in this sequence, and because tau is multiplicative: if a and b are in this sequence and (a,b)=1 then a*b is also in a(n). - Enrique Pérez Herrero, Jan 22 2013
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REFERENCES
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S. Colton, Automated Theorem Discovery: A Future Direction for Theorem Provers, 2002.
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LINKS
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EXAMPLE
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tau(6)=4, which is a square number, so 6 is in this sequence.
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MATHEMATICA
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Select[Range[200], IntegerQ[Sqrt[DivisorSigma[0, #]]]&] (* Harvey P. Dale, Apr 20 2011 *)
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PROG
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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Simon Colton (simonco(AT)cs.york.ac.uk)
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EXTENSIONS
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STATUS
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approved
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