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A055008 Numbers k such that gcd(phi(k), sigma(k)) = 1 with phi = A000010, sigma = A000203. 6
1, 2, 4, 8, 9, 16, 25, 32, 36, 50, 64, 81, 100, 121, 128, 144, 225, 242, 256, 289, 324, 400, 484, 512, 529, 576, 578, 625, 729, 800, 841, 900, 1024, 1058, 1089, 1156, 1250, 1296, 1600, 1681, 1682, 1936, 2025, 2048, 2116, 2209, 2304, 2312, 2401, 2500, 2601 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The asymptotic density of this sequence is 0 (Dressler, 1974). - Amiram Eldar, Jul 23 2020

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

Robert E. Dressler, On a theorem of Niven, Canadian Mathematical Bulletin, Vol. 17, No. 1 (1974), pp. 109-110.

EXAMPLE

For n = 484, phi(484) = 220 = 2*2*5*11, sigma(484) = 931 = 7*7*19, and gcd(220,931) = 1.

MATHEMATICA

Select[Range@ 2700, CoprimeQ[EulerPhi@ #, DivisorSigma[1, #]] &] (* Michael De Vlieger, Feb 05 2017 *)

PROG

(PARI) is(n)=gcd(sigma(n), eulerphi(n))==1 \\ Charles R Greathouse IV, Feb 19 2013

CROSSREFS

Cf. A000010, A000203, A009223.

Sequence in context: A251642 A079931 A188915 * A204826 A241010 A354956

Adjacent sequences:  A055005 A055006 A055007 * A055009 A055010 A055011

KEYWORD

nonn

AUTHOR

Labos Elemer, May 31 2000

EXTENSIONS

Incorrect comment removed by Charles R Greathouse IV, Feb 19 2013

STATUS

approved

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Last modified October 1 19:19 EDT 2022. Contains 357150 sequences. (Running on oeis4.)