A067230 Numbers n such that phi(2n-1) = sigma(n).

1, 3, 5, 10, 11, 23, 33, 46, 65, 77, 123, 136, 221, 371, 383, 423, 483, 513, 545, 670, 753, 1643, 2033, 3838, 3885, 4533, 4798, 5177, 7463, 7739, 8201, 8985, 9010, 9393, 12256, 13144, 14104, 17110, 17147, 17663, 18695, 19817, 21119, 23038, 24695

Offset: 1

Author

Benoit Cloitre, Feb 20 2002

Keywords

• nonn

Links

• Donovan Johnson, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000010, A000203, A067229.

Programs

Mathematica

Select[Range[25000],EulerPhi[2#-1]==DivisorSigma[1,#]&] (* Harvey P. Dale, Aug 22 2015 *)

PARI

isok(n) = eulerphi(2*n-1) == sigma(n); \\ Michel Marcus, Nov 21 2013