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 A066175 Numbers n such that sigma(phi(sigma(n))) = n. 2
 1, 3, 7, 15, 31, 127, 1023, 8191, 131071, 524287, 2147483647 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 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? It is conjectured that there are infinitely many Mersenne primes. So this conjecture also supports that this sequence is infinite. Additionally, if n=2^k-1, where either k=1, or n is a Mersenne prime (A000668), or sigma(n)=3*2^(k-1), then A000217(n) divides sigma(A000217(n)). - Altug Alkan, Jul 25 2016 LINKS EXAMPLE sigma(phi(sigma(31))) = sigma(phi(32)) = sigma(16) = 31. MATHEMATICA Select[Range[1, 10^6], DivisorSigma[1, EulerPhi[DivisorSigma[1, # ]]]==#&] CROSSREFS Sequence in context: A147094 A147285 A147250 * A132978 A117079 A026745 Adjacent sequences:  A066172 A066173 A066174 * A066176 A066177 A066178 KEYWORD nonn,more AUTHOR Joseph L. Pe, Dec 15 2001 EXTENSIONS Edited by Dean Hickerson, Feb 20 2002 a(11) from Jud McCranie, Jun 23 2005; no more terms < 4000000000 STATUS approved

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Last modified May 26 13:05 EDT 2019. Contains 323586 sequences. (Running on oeis4.)