A193167 Composite numbers k such that phi(k) divides 4*(k-1).

4, 6, 8, 10, 12, 15, 28, 66, 91, 561, 946, 6601, 8911, 6840001

Offset

1,1

Comments

From Amiram Eldar, May 28 2024: (Start)

The terms 561, 6601, 8911 and 6840001 are Carmichael numbers (A002997). The next Carmichael number in this sequence is 16187797671051601, and there are no more Carmichael numbers in this sequence below 10^22.

a(15) > 10^10. (End)

Links

Table of n, a(n) for n=1..14.

Mathematica

t={}; Do[If[PrimeQ[n]==False && IntegerQ[(n-1)*4/EulerPhi[n]], Print[n]; AppendTo[t, n]], {n, 2, 10^5}]; t
Select[Range[7*10^6], CompositeQ[#]&&Divisible[4(#-1), EulerPhi[#]]&] (* Harvey P. Dale, Jul 07 2016 *)

Prog

(PARI) p=1; forprime(q=2, 1e9, for(n=p+1, q-1, if((4*n-4)%eulerphi(n)==0, print1(n", "))); p=q) \\ Charles R Greathouse IV, Jul 31 2011

Crossrefs

Cf. A000010, A002997.

Sequence in context: A286043 A310658 A167179 * A020645 A053226 A227112

Adjacent sequences: A193164 A193165 A193166 * A193168 A193169 A193170

Keyword

nonn,more

Author

José María Grau Ribas, Jul 20 2011

Status

approved