A015840 Numbers k such that phi(k) | sigma(k + 3).

1, 2, 3, 4, 7, 8, 12, 14, 19, 20, 21, 24, 27, 30, 32, 39, 52, 63, 84, 91, 102, 112, 117, 127, 129, 132, 135, 155, 165, 181, 200, 210, 252, 327, 348, 387, 403, 405, 410, 432, 452, 453, 456, 459, 475, 480, 532, 549, 592, 620, 624, 679, 693, 714, 725, 952, 1009

Offset: 1

Author

Robert G. Wilson v

Keywords

• nonn

Links

• Ivan Neretin, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A020492.

Programs

Maple

with(numtheory): A015840:=n->`if`(sigma(n+3) mod phi(n) = 0, n, NULL): seq(A015840(n), n=1..5*10^3); # Wesley Ivan Hurt, Feb 04 2017

Mathematica

Select[Range[3000], Divisible[DivisorSigma[1, 3 + #], EulerPhi[#]] &] (* David Nacin, Mar 03 2012 *)

PARI

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

Extensions

a(1) = 1 inserted by Ivan Neretin, Jan 30 2017