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A065523 Numbers n such that sigma(n) is a prime power (A025475). 1
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

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

MATHEMATICA

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

CROSSREFS

A proper subset of A065496.

Sequence in context: A323585 A003585 A108102 * A046528 A018572 A018641

Adjacent sequences:  A065520 A065521 A065522 * A065524 A065525 A065526

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Nov 27 2001

EXTENSIONS

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

STATUS

approved

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Last modified January 18 16:27 EST 2020. Contains 331011 sequences. (Running on oeis4.)