A056773 - OEIS

%I #10 Apr 28 2020 15:31:19

%S 12,18,24,28,36,40,66,88,124,184,232,328,424,508,664,712,904,1048,

%T 1384,1432,1528,1864,1912,2008,2248,2344,2586,2872,3352,3448,3544,

%U 3928,4072,4744,5128,5224,5272,5464,5752,5944,6088,6472,7288,7624,8104,8152,8248

%N Composite n such that phi(n+4) = phi(n)+4.

%C Are all terms even? - _Robert Israel_, Apr 28 2020

%H Robert Israel, <a href="/A056773/b056773.txt">Table of n, a(n) for n = 1..10000</a>

%e 24 is in the sequence because 24 is composite and phi(24)+4 = 12 = phi(24+4).

%p filter:= n -> not isprime(n) and numtheory:-phi(n+4)=numtheory:-phi(n)+4:

%p select(filter, [$1..10000]); # _Robert Israel_, Apr 28 2020

%t Select[Range[9000],CompositeQ[#]&&EulerPhi[#]+4==EulerPhi[#+4]&] (* _Harvey P. Dale_, Feb 12 2015 *)

%o (PARI) is(n)=!isprime(n) && eulerphi(n+4)==eulerphi(n)+4 \\ _Charles R Greathouse IV_, Apr 28 2020

%Y A001838, A015913, A055458. Composites in A056772. Primes in A056772 are A023200.

%K nonn

%O 1,1

%A _Labos Elemer_, Aug 17 2000

%E Edited by _Robert Israel_, Apr 28 2020