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Primes p such that phi(p+1) = phi(p-1).
3

%I #18 Sep 16 2022 12:40:28

%S 5,11,71,911,1609,2591,4339,15401,58309,59149,80989,208391,215389,

%T 496511,589189,1809079,1970149,3167569,3260809,3516109,5914369,

%U 9832271,12231311,13608071,14470061,16537151,16692551,19018369,19462661

%N Primes p such that phi(p+1) = phi(p-1).

%C Primes in A066812.

%H Giovanni Resta, <a href="/A067890/b067890.txt">Table of n, a(n) for n = 1..675</a>

%F a(n) = prime(A066902(n)). - _Giovanni Resta_, Apr 06 2020

%t Select[Prime[Range[124*10^4]],EulerPhi[#-1]==EulerPhi[#+1]&] (* _Harvey P. Dale_, Apr 09 2019 *)

%o (PARI) v=[]; forprime(p=2,40000000,if(eulerphi(p+1)==eulerphi(p-1),v=concat(v,p))); v

%Y Cf. A066902, A066812.

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, Mar 02 2002

%E More terms from _Rick L. Shepherd_, May 03 2002