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A056772
Numbers n such that phi(n+4) = phi(n) + 4.
6
3, 7, 12, 13, 18, 19, 24, 28, 36, 37, 40, 43, 66, 67, 79, 88, 97, 103, 109, 124, 127, 163, 184, 193, 223, 229, 232, 277, 307, 313, 328, 349, 379, 397, 424, 439, 457, 463, 487, 499, 508, 613, 643, 664, 673, 712, 739, 757, 769, 823, 853, 859, 877, 883, 904, 907
OFFSET
1,1
COMMENTS
In contrast with A015913, composite solutions are not rare. Prime solutions are common.
From Kevin J. Gomez, Mar 02 2016: (Start)
Composite solutions have two known forms:
n such that n = 4 * (2^p - 1) where 2^p - 1 is a Mersenne prime. (A001348)
n such that n = 8q where q is a Sophie Germain prime. (A005394)
There are composite solutions (such as 36) that do not fit either of these forms.
(End)
LINKS
EXAMPLE
n=1048: phi(1048)=520, phi(1048+4)=524.
MATHEMATICA
Select[Range@1000, EulerPhi@(# + 4)== EulerPhi[#] + 4 &] (* Vincenzo Librandi, Sep 11 2015 *)
Position[Partition[EulerPhi[Range[1000]], 5, 1], _?(#[[1]]+4==#[[5]]&), 1, Heads-> False]//Flatten (* Harvey P. Dale, Dec 18 2019 *)
PROG
(PARI) isok(n) = eulerphi(n+4) == eulerphi(n) + 4; \\ Michel Marcus, Sep 11 2015
(Magma) [n: n in [1..1000] | EulerPhi(n+4) eq EulerPhi(n)+4]; // Vincenzo Librandi, Sep 11 2015
CROSSREFS
Cf. A015913 (sigma(n+4) = sigma(n) + 4).
Cf. A001838 (k=2), this sequence (k=4), A262084 (k=6), A262085 (k=8), A262086 (k=10).
Sequence in context: A243705 A057927 A298788 * A024614 A230109 A045134
KEYWORD
nonn
AUTHOR
Labos Elemer, Aug 17 2000
STATUS
approved