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%I #12 Sep 08 2022 08:45:08
%S 5,7,8,9,10,12,13,14,15,16,18,19,20,21,24,25,26,27,28,30,31,33,35,36,
%T 38,39,42,43,44,45,49,50,51,52,54,56,61,62,64,65,66,68,69,70,72,73,77,
%U 78,80,81,84,86,90,91,92,93,95,96,98,99,102,103,104,105,109,111,112,117
%N Numbers n such that phi(n) - 1 is prime, where phi is Euler's totient function (A000010).
%C For all primes p: p is in the sequence iff p is the greater member of a twin prime pair (A006512), see A078893.
%C Union of A006512 and A078893. - _Ray Chandler_, May 26 2008
%H Vincenzo Librandi, <a href="/A078892/b078892.txt">Table of n, a(n) for n = 1..1000</a>
%t Select[Range[200], PrimeQ[EulerPhi[#] - 1]&] (* _Vincenzo Librandi_, Aug 13 2013 *)
%o (PARI) is(n)=isprime(eulerphi(n)-1) \\ _Charles R Greathouse IV_, Feb 21 2013
%o (Magma) [n: n in [1..200] | IsPrime(EulerPhi(n)-1)]; // _Vincenzo Librandi_, Aug 13 2013
%Y Cf. A000010, A000040, A008864, A109606, A006512, A039698, A072281, A078893.
%K nonn,easy
%O 1,1
%A _Reinhard Zumkeller_, Dec 12 2002
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