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A326181
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Numbers n for which sigma(sigma(n)) = 3*sigma(n).
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5
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54, 56, 87, 95, 276, 308, 429, 446, 455, 501, 581, 611, 158928, 194928, 195072, 199950, 226352, 234608, 236432, 248325, 255678, 263504, 266192, 273050, 275415, 304575, 336903, 341162, 353675, 366575, 369425, 369843, 380463, 386313, 389463, 406565, 411725, 415925, 422303, 447587, 468743, 497333, 500993, 511829, 515267, 519557, 519677
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OFFSET
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1,1
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COMMENTS
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Any odd perfect numbers must occur in this sequence, as such numbers must be in the intersection of A000396 and A326051, that is, satisfy both sigma(n) = 2n and sigma(2n) = 6n = 3*2n, thus in combination they must satisfy sigma(sigma(n)) = 3*sigma(n). Note that odd perfect numbers should occur also in A019283.
If, as conjectured, A005820 has 6 terms, then this sequence is finite and has 756 terms. - Giovanni Resta, Jun 17 2019
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LINKS
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PROG
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(PARI) isA326181(n) = { my(s=sigma(n)); (sigma(s)==3*s); };
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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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