A193167 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

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

(list; graph; refs; listen; history; text; internal format)

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