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 A066881 Numbers n such that sigma(phi(n))/sigma(n) is an integer >= 4. 2
 121679, 1043909, 2350171, 2918263, 3396103, 3566807, 3688067, 4019467, 4562827, 5963407, 7300697, 7485979, 7853933, 8103301, 8364151, 9237779, 9514213, 9638527, 10531123, 11094619, 11384447, 12721937, 13576267, 15331313 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: all natural numbers 1,2,3,...,n will eventually occur among the integer values of sigma(phi(n))/sigma(n). LINKS Harry J. Smith, Table of n, a(n) for n = 1..370 EXAMPLE a(1) = 121679 because sigma(phi(121679))/sigma(121679) = 4. MAPLE for n from 1 do spn := numtheory[sigma](numtheory[phi](n)) ; sn := numtheory[sigma](n) ; if spn mod sn = 0 then if spn/sn >= 4 then print(n, spn/sn) fi; fi; od: # R. J. Mathar, Oct 01 2008 PROG (PARI) { n=0; for (m=1, 10^10, if((e=sigma(eulerphi(m))) % (s=sigma(m)) == 0 && e/s >= 4, write("b066881.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Apr 03 2010 CROSSREFS Cf. A066817. Cf. A067383, A067382. - R. J. Mathar, Oct 01 2008 Sequence in context: A236826 A135495 A067384 * A048903 A237793 A185864 Adjacent sequences: A066878 A066879 A066880 * A066882 A066883 A066884 KEYWORD nonn AUTHOR Jason Earls, Jan 22 2002 EXTENSIONS Terms a(16) etc. from R. J. Mathar, Oct 01 2008 STATUS approved

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Last modified May 31 19:08 EDT 2023. Contains 363068 sequences. (Running on oeis4.)