A107815 - OEIS

%I #4 Sep 01 2021 11:31:43

%S 2,17,67,211,631,1051,1811,1951,2287,2791,3331,7297,10099,13597,15739,

%T 19717,21169,22153,41341,85933,400051,2251201,2991253,3051751,4244791,

%U 5713891

%N Primes p such that sigma(k) = phi(prime(k)-1), where p = prime(k).

%C a(n) = prime(A067651(n)).

%C Values of k are in A067651, values of sigma(k) are in A107816.

%e a(3) = prime(A067651(3)) = prime(19) = 67.

%t Prime[#]&/@Select[Range[400000],DivisorSigma[1,#]==EulerPhi[Prime[#]-1]&] (* _Harvey P. Dale_, Sep 01 2021 *)

%o (PARI) m=400000;for(n=1,m,p=prime(n);if(sigma(n)==eulerphi(p-1),print1(p,",")))

%Y Cf. A067651, A107816.

%K nonn,more

%O 1,1

%A _Klaus Brockhaus_, May 24 2005