A015849 Numbers n such that phi(n + 9) | sigma(n) for n not congruent to 0 (mod 3).

5, 7, 10, 31, 47, 79, 127, 145, 161, 223, 238, 239, 355, 367, 371, 376, 418, 455, 463, 479, 748, 863, 1039, 1045, 1087, 1103, 1118, 1327, 1423, 1439, 1567, 1583, 1823, 1886, 1999, 2065, 2108, 2143, 2201, 2207, 2239, 2447, 2461, 2687, 2767, 2840, 2927, 2975

Offset: 1

Author

Robert G. Wilson v

Keywords

• nonn

Comments

Includes primes p such that (p+9)/8 is prime. Thus Dickson's conjecture implies the sequence is infinite. - Robert Israel, Jan 10 2019

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A015827.

Programs

Maple

select(n -> (numtheory:-sigma(n)/numtheory:-phi(n+9))::integer, [seq(seq(3*i+j,j=1..2),i=0..1000)]); # Robert Israel, Jan 10 2019

Mathematica

Select[Range[1, 5000], Divisible[DivisorSigma[1, #], EulerPhi[9 + #]] && ! Mod[#, 3] == 0 &] (* David Nacin, Mar 04 2012 *)

PARI

is(n)=n%3 && sigma(n)%eulerphi(n+9)==0 \\ Charles R Greathouse IV, Sep 25 2012