A065523 Numbers n such that sigma(n) is a prime power (A025475).

3, 7, 21, 31, 81, 93, 127, 217, 381, 400, 651, 889, 2667, 3937, 8191, 11811, 24573, 27559, 57337, 82677, 131071, 172011, 253921, 393213, 524287, 761763, 917497, 1040257, 1572861, 1777447, 2752491, 3120771, 3670009, 4063201, 5332341

Offset: 1

Author

Robert G. Wilson v, Nov 27 2001

Keywords

• nonn

Links

• Donovan Johnson, Table of n, a(n) for n = 1..80

Crossrefs

A proper subset of A065496.

Programs

Mathematica

s[n_] := DivisorSigma[1, n ]; Select[ Range[2, 10^6], !PrimeQ[ s[ # ]] && Mod[ s[ # ], s[ # ] - EulerPhi[ s[ # ]]] == 0 & ]

Extensions

a(31)-a(35) from Donovan Johnson, Sep 05 2008