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A067236
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Numbers n such that gcd(sigma(n), sigma(sigma(n)))=1.
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1
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1, 2, 3, 4, 7, 9, 16, 18, 21, 22, 25, 31, 32, 49, 50, 52, 64, 66, 70, 81, 93, 94, 97, 98, 100, 115, 119, 121, 127, 144, 156, 162, 169, 170, 196, 210, 214, 217, 225, 241, 242, 256, 265, 282, 289, 291, 310, 322, 337, 338, 343, 345, 357, 361, 364, 381, 382, 385, 400
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OFFSET
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1,2
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COMMENTS
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LINKS
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MATHEMATICA
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ds1Q[n_]:=Module[{ds=DivisorSigma[1, n]}, GCD[ds, DivisorSigma[1, ds]]==1]; Select[Range[500], ds1Q] (* Harvey P. Dale, Jun 13 2014 *)
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PROG
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(PARI) isok(n) = gcd(sigma(n), sigma(sigma(n))) == 1; \\ Michel Marcus, Feb 08 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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