A257550 Numbers k such that phi(k) = 5*phi(k+1).

17907119, 18828809, 31692569, 73421039, 179467469, 322757819, 337567229, 627702389, 975314339, 2537636009, 2722271369, 3328653509, 3917646809, 5529412349, 6369847469, 11179199849, 11201693579, 11363832479, 13442120999, 16781760449, 19751331599, 20002320029

Offset

1,1

Comments

All terms < 10^15 are congruent to 9 mod 10 except a(918) = 876200950266641. - Jud McCranie, Jan 22 2026

Links

Jud McCranie, Table of n, a(n) for n = 1..943 (terms 1..103 from Giovanni Resta)

Example

phi(17907119) = 16588800 = 5*phi(17907120).

Mathematica

a1={}; nmax=10^9; last=EulerPhi[1]; n=2;
While[n<nmax, ep=EulerPhi[n];
If[last==5ep, AppendTo[a1, n-1]; ];
n++; last=ep; ]; a1 (* Ray Chandler, Apr 30 2015 *)

Crossrefs

Cf. A001274, A171262, A256907, A256937.

Sequence in context: A154875 A015353 A083619 * A282670 A238071 A271445

Adjacent sequences: A257547 A257548 A257549 * A257551 A257552 A257553

Keyword

nonn

Author

Ray Chandler, Apr 29 2015

Extensions

a(10)-a(22) from Giovanni Resta, May 11 2015

Status

approved