|
%I #15 Jun 03 2022 03:20:32
%S 19,47,77,79,115,177,191,267,280,297,340,398,406,446,470,639,839,917,
%T 1240,1655,1713,1778,1836,2140,2157,4715,5110,5269,6585,7089,7965,
%U 8517,8844,9785,10281,12586,15279,16100,16468,16732,17724,18145,18256,20728
%N Numbers k such that phi((prime(k)-1)/2) = sigma(k).
%H Amiram Eldar, <a href="/A068474/b068474.txt">Table of n, a(n) for n = 1..300</a> (terms below 10^9, terms 1..130 from G. C. Greubel)
%t Select[Range[2, 3000], EulerPhi[(Prime[#]-1)/2]==DivisorSigma[1, #]&] (* _G. C. Greubel_, Oct 13 2018 *)
%o (PARI) for(n=2,45000, if(eulerphi((prime(n)-1)/2)==sigma(n),print1(n,",")))
%Y Cf. A000010, A000203.
%K easy,nonn
%O 1,1
%A _Benoit Cloitre_, Mar 10 2002
|
