A039771 - OEIS

A039771

Numbers k such that phi(k) is a perfect cube.

1, 2, 15, 16, 20, 24, 30, 85, 128, 136, 160, 170, 192, 204, 240, 247, 259, 327, 333, 351, 399, 405, 436, 494, 518, 532, 648, 654, 666, 684, 702, 756, 771, 798, 810, 1024, 1028, 1088, 1111, 1255, 1280, 1360, 1375, 1536, 1542, 1632, 1843, 1853, 1875

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OFFSET

1,2

COMMENTS

a(n) is prime only for a(2)=2, for other cases: eulerphi(p) = p-1 = n^3, and p = 1 + n^3 = (n+1)(n^2-n+1), so p cannot be a prime. - Enrique Pérez Herrero, Aug 29 2010

A013730 is a subsequence. - Enrique Pérez Herrero, Aug 29 2010

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..2500 from E. Pérez Herrero)

EXAMPLE

phi(247) = 216 = 6*6*6.

MATHEMATICA

Select[ Range[ 2000 ], IntegerQ[ Power[ EulerPhi[ # ], 1/3 ] ]& ]

PROG

(PARI) for(n=1, 1e4, if(ispower(eulerphi(n), 3), print1(n", "))) \\ Charles R Greathouse IV, Jul 31 2011

CROSSREFS

Cf. A000010, A007614, A039770, A216412.

Sequence in context: A037312 A267711 A108472 * A032934 A370977 A337824

Adjacent sequences: A039768 A039769 A039770 * A039772 A039773 A039774

KEYWORD

nonn

AUTHOR

Olivier Gérard

STATUS

approved