A056773 Composite n such that phi(n+4) = phi(n)+4.

12, 18, 24, 28, 36, 40, 66, 88, 124, 184, 232, 328, 424, 508, 664, 712, 904, 1048, 1384, 1432, 1528, 1864, 1912, 2008, 2248, 2344, 2586, 2872, 3352, 3448, 3544, 3928, 4072, 4744, 5128, 5224, 5272, 5464, 5752, 5944, 6088, 6472, 7288, 7624, 8104, 8152, 8248

Offset

1,1

Comments

Are all terms even? - Robert Israel, Apr 28 2020

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

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

Maple

filter:= n -> not isprime(n) and numtheory:-phi(n+4)=numtheory:-phi(n)+4:
select(filter, [$1..10000]); # Robert Israel, Apr 28 2020

Mathematica

Select[Range[9000], CompositeQ[#]&&EulerPhi[#]+4==EulerPhi[#+4]&] (* Harvey P. Dale, Feb 12 2015 *)

Prog

(PARI) is(n)=!isprime(n) && eulerphi(n+4)==eulerphi(n)+4 \\ Charles R Greathouse IV, Apr 28 2020

Crossrefs

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

Sequence in context: A280014 A359929 A162151 * A297925 A341099 A175837

Adjacent sequences: A056770 A056771 A056772 * A056774 A056775 A056776

Keyword

nonn

Author

Labos Elemer, Aug 17 2000

Extensions

Edited by Robert Israel, Apr 28 2020

Status

approved