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 A015762 Numbers n such that phi(n) | sigma_4(n). 11
 1, 2, 3, 6, 249, 498 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS sigma_4(n) is the sum of the 4th powers of the divisors of n (A001159). sigma_{8j+4}(x)/phi(x) is an integer for j=0..500, x=1,2,3,6,249,498, and this is conjectured to hold for possible larger terms of A015762 and all j. Compare with comments to A015759, A091285, A015770. - Labos Elemer, May 27 2004 For any odd n in this sequence, 2n is also in the sequence, since phi(2n) = phi(n) and sigma_4(2n) = 17 sigma_4(n). More generally, if gcd(m,n) = 1 and m and n both are in this sequence, then mn is also in the sequence. No odd prime > 3 can be in the sequence, since if p = 2r + 1, then sigma_4(p) = 8r(2r^3 + 4r^2 + 3r + 1) + 2 is divisible by phi(p) = 2r only for r = 1. The term a(5) = 3*83 is the only odd semiprime term with a factor < 10^5. - M. F. Hasler, Aug 21 2017 a(7) > 3*10^11, if it exists. - Giovanni Resta, Aug 23 2017 LINKS MATHEMATICA Select[Range[500], Divisible[DivisorSigma[4, #], EulerPhi[#]]&] (* Harvey P. Dale, Dec 16 2012 *) PROG (PARI) select( is(n)=sigma(n, 4)%eulerphi(n)==0, [1..10^4])  \\ M. F. Hasler, Aug 21 2017 (MAGMA) [n: n in [1..1000]| DivisorSigma(4, n) mod EulerPhi(n) eq 0]; // Vincenzo Librandi, Aug 22 2017 CROSSREFS Cf. A020492, A015759, A015761, A015763, A015764, A015765, A015766, A015767, A015768, A015769, A015770, A015771, A015773, A015774, A094470. Sequence in context: A018755 A018769 A297624 * A015770 A093038 A094469 Adjacent sequences:  A015759 A015760 A015761 * A015763 A015764 A015765 KEYWORD nonn AUTHOR STATUS approved

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Last modified August 2 02:53 EDT 2021. Contains 346409 sequences. (Running on oeis4.)