A078893 - OEIS

%I #13 Jan 01 2022 09:55:49

%S 8,9,10,12,14,15,16,18,20,21,24,25,26,27,28,30,33,35,36,38,39,42,44,

%T 45,49,50,51,52,54,56,62,64,65,66,68,69,70,72,77,78,80,81,84,86,90,91,

%U 92,93,95,96,98,99,102,104,105,111,112,117,120,121,122,123,124,129,130,133

%N Composite numbers k such that phi(k) - 1 is prime, where phi is Euler's totient function (A000010).

%C A078892 with the primes removed. - _Ray Chandler_, May 26 2008

%H Amiram Eldar, <a href="/A078893/b078893.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[150],CompositeQ[#]&&PrimeQ[EulerPhi[#]-1]&] (* _Harvey P. Dale_, Dec 28 2021 *)

%o (PARI) is(n)=!isprime(n) && isprime(eulerphi(n)-1) \\ _Charles R Greathouse IV_, Feb 21 2013

%Y Cf. A000010, A000040, A008864, A109606, A078892, A066071, A068019.

%K nonn

%O 1,1

%A _Reinhard Zumkeller_, Dec 12 2002