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

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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
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
KEYWORD
nonn
AUTHOR
Jason Earls, Jan 22 2002
EXTENSIONS
Terms a(16) etc. from R. J. Mathar, Oct 01 2008
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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 05:39 EDT 2024. Contains 371235 sequences. (Running on oeis4.)