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A015730
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Numbers n such that tau(sigma(n))= tau(tau(n)).
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1
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1, 2, 4, 9, 16, 18, 25, 50, 64, 144, 289, 576, 578, 729, 1458, 1600, 1681, 2401, 2916, 3362, 3481, 3600, 4096, 4624, 4802, 5041, 6962, 7921, 9604, 10082, 10201, 11664, 15625, 15842, 17161, 18225, 18496, 20402, 21609, 24400, 26896, 27889
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OFFSET
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1,2
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LINKS
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MATHEMATICA
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Do[ If[ DivisorSigma[ 0, DivisorSigma[ 0, n ] ] == DivisorSigma[ 0, DivisorSigma[ 1, n ] ], Print[ n ] ], {n, 1, 50000} ]
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PROG
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(PARI) isok(n) = numdiv(sigma(n)) == numdiv(numdiv(n)); \\ Michel Marcus, Aug 12 2017
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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