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!)
A015765 Numbers n such that phi(n) | sigma_7(n). 12
1, 2, 3, 6, 12, 14, 15, 30, 35, 42, 56, 70, 78, 105, 140, 168, 190, 210, 248, 264, 270, 295, 357, 418, 420, 570, 590, 594, 616, 630, 714, 744, 767, 812, 840, 885, 910, 1038, 1045, 1240, 1254, 1416, 1485, 1534, 1589, 1672, 1770, 1848, 2065, 2090, 2214, 2301, 2376, 2422, 2436, 2580, 2730, 2970 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

sigma_7(n) is the sum of the 7th powers of the divisors of n.

In contrast to other values of k for which sigma_k(n) is much less frequently divisible by phi(n) (cf. A015762 (k=4: a(7) > 10^11 if it exists), A015759 (k=2: a(23) > 10^11)), it is quite easy to compute hundreds or even thousands of terms of the present sequence. - M. F. Hasler, Aug 26 2017

LINKS

M. F. Hasler, Table of n, a(n) for n = 1..10000 (first 1001 terms from Vincenzo Librandi)

MATHEMATICA

Select[Range[2100], Divisible[DivisorSigma[7, #], EulerPhi[#]]&] (* Harvey P. Dale, Aug 17 2013 *)

PROG

(PARI) select( is(n)=sigma(n, 7)%eulerphi(n)==0, [1..3000]) \\ M. F. Hasler, Aug 26 2017

CROSSREFS

Sequence in context: A015761 A015763 A015769 * A015771 A020492 A110590

Adjacent sequences:  A015762 A015763 A015764 * A015766 A015767 A015768

KEYWORD

nonn,easy

AUTHOR

Robert G. Wilson v

EXTENSIONS

Third line of data completed by M. F. Hasler, Aug 26 2017

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 30 23:12 EDT 2021. Contains 346365 sequences. (Running on oeis4.)