The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A066175 Numbers k such that sigma(phi(sigma(k))) = k. 4
 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 Table of n, a(n) for n=1..11. EXAMPLE sigma(phi(sigma(31))) = sigma(phi(32)) = sigma(16) = 31. MATHEMATICA Select[Range[1, 10^6], DivisorSigma[1, EulerPhi[DivisorSigma[1, # ]]]==#&] CROSSREFS Cf. A000217, A000668. Sequence in context: A147285 A147250 A336701 * A132978 A336976 A117079 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified June 9 00:56 EDT 2023. Contains 363165 sequences. (Running on oeis4.)