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%I #18 Apr 24 2022 06:36:54
%S 13,43,109,151,589,883,2143,2725,4825,4921,9541,13189,21637,22249,
%T 22489,29971,30229,33787,36247,72541,73513,83287,94489,109213,113269,
%U 117367,189103,190489,198457,216529,247597,277447,297307,320137,353821,357751,376753,391543
%N Numbers k such that sigma(k+1) = 2*phi(k).
%C If p=2^n+3 and both numbers p & q=(1/2)*(p^2-3p-2) are primes then q is in the sequence, because sigma(q+1)=sigma((1/2)*(p-3)*p)= sigma(2^(n-1)*p)=(2^n-1)*(p+1)=(p-4)*(p+1)=p^2-3p-4=2q-2=2*phi(q). 13, 43, 151, 2143 & 34360131583 are such terms corresponding to n = 2, 3, 4, 6 & 18. - _Farideh Firoozbakht_, Feb 16 2008
%H Amiram Eldar, <a href="/A067260/b067260.txt">Table of n, a(n) for n = 1..1000</a>
%t Do[If[DivisorSigma[1,n+1]==2*EulerPhi@n,Print[n]],{n,200000}] (* _Farideh Firoozbakht_, Feb 16 2008 *)
%Y Cf. A000010, A000203, A067261, A067262, A067263, A135241.
%K nonn
%O 1,1
%A _Benoit Cloitre_, Feb 21 2002
%E More terms from _Amiram Eldar_, Apr 24 2022
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