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A066175
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Numbers n such that sigma(phi(sigma(n))) = n.
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1
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1, 3, 7, 15, 31, 127, 1023, 8191, 131071, 524287, 2147483647
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| If n=2^k-1, where either k=1, or n is a Mersenne prime (A000668), or sigma(n)=3*2^(k-1), then n is in the sequence; are there any terms not of these forms? The last form includes the terms 15 and 1023; are there others like this?
Is this sequence infinite?
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EXAMPLE
| sigma(phi(sigma(31))) = sigma(phi(32)) = sigma(16) = 31.
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MATHEMATICA
| Select[Range[1, 10^6], DivisorSigma[1, EulerPhi[DivisorSigma[1, # ]]]==#&]
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CROSSREFS
| Sequence in context: A147094 A147285 A147250 * A132978 A117079 A026745
Adjacent sequences: A066172 A066173 A066174 * A066176 A066177 A066178
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KEYWORD
| nonn,more
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AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Dec 15 2001
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EXTENSIONS
| Edited by Dean Hickerson (dean.hickerson(AT)yahoo.com), Feb 20 2002
a(11) from Jud McCranie, Jun 23, 2005. No more terms < 4000000000.
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