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Numbers k such that phi(k) | sigma(k + 3).
6

%I #26 Feb 12 2021 01:30:28

%S 1,2,3,4,7,8,12,14,19,20,21,24,27,30,32,39,52,63,84,91,102,112,117,

%T 127,129,132,135,155,165,181,200,210,252,327,348,387,403,405,410,432,

%U 452,453,456,459,475,480,532,549,592,620,624,679,693,714,725,952,1009

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

%H Ivan Neretin, <a href="/A015840/b015840.txt">Table of n, a(n) for n = 1..1000</a>

%p 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

%t Select[Range[3000], Divisible[DivisorSigma[1, 3 + #], EulerPhi[#]] &] (* _David Nacin_, Mar 03 2012 *)

%o (PARI) is(n)=!(sigma(n+3)%eulerphi(n)) \\ _Charles R Greathouse IV_, Sep 25 2012

%Y Cf. A020492.

%K nonn

%O 1,2

%A _Robert G. Wilson v_

%E a(1) = 1 inserted by _Ivan Neretin_, Jan 30 2017