login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A067740 Smallest number k such that sigma(k)/sigma(phi(k)) = n. 1
1, 14, 2, 6, 118740, 2915640, 74322349920 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The quotient sigma(k)/sigma(phi(k)) is integral for the numbers in A190503. Does a(n) exist for all n?

10^11 < a(8) <= 11224976029787520. - Donovan Johnson, Jun 07 2011

LINKS

Table of n, a(n) for n=1..7.

FORMULA

a(n)=Min{x; A000203(x)/A000203[A000010(x)]=n}

EXAMPLE

n=6, a(6)=2915640, sigma(2915640)=11793600, phi(2915640)=608256, sigma(608256)=1965600 and 11793600=6*1965600.

MATHEMATICA

g[x_] := DivisorSigma[1, x] / DivisorSigma[1, EulerPhi[x]]; m=10; up=200000; a = Table[0, {m}]; Do[ b = g[n]; If[b <= m && IntegerQ[b] && a[[b]] == 0, a[[b]] = n], {n, 1, up} ]; a

CROSSREFS

Cf. A000203, A000010, A067385, A190503.

Sequence in context: A040194 A107834 A070701 * A037923 A040195 A303380

Adjacent sequences:  A067737 A067738 A067739 * A067741 A067742 A067743

KEYWORD

nonn,more

AUTHOR

Labos Elemer, Jan 29 2002

EXTENSIONS

a(7) from Donovan Johnson, Jun 07 2011

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 12 08:23 EDT 2020. Contains 335657 sequences. (Running on oeis4.)