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A067612 Numbers n such that sigma(n) = 3*phi(sigma(n)). 1
5, 6, 10, 11, 14, 15, 17, 22, 23, 30, 33, 34, 35, 42, 46, 47, 51, 53, 55, 62, 66, 69, 70, 71, 77, 85, 94, 102, 105, 106, 107, 110, 115, 119, 138, 141, 142, 154, 155, 159, 161, 165, 170, 186, 187, 191, 210, 213, 214, 230, 231, 235, 238, 253, 254, 255, 265, 282, 310 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

From Robert Israel, Feb 26 2017: (Start)

Numbers n such that sigma(n) is in A033845.

Contains A139257. (End)

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

MAPLE

filter:= proc(n) local s; s:= numtheory:-sigma(n); s mod 6 = 0 and s = 2^padic:-ordp(s, 2)*3^padic:-ordp(s, 3) end proc:

select(filter, [$1..10000]); # Robert Israel, Feb 26 2017

MATHEMATICA

s3pQ[n_]:=Module[{s=DivisorSigma[1, n]}, s==3*EulerPhi[s]]; Select[ Range[ 400], s3pQ] (* Harvey P. Dale, May 28 2015 *)

PROG

(PARI) for(n=1, 500, if(sigma(n)==3*eulerphi(sigma(n)), print1(n, ", "))) \\ Derek Orr, Feb 26 2017

(PARI) is(n)=my(s=sigma(n)); s%3==0 && s==3*eulerphi(s) \\ Charles R Greathouse IV, Feb 27 2017

CROSSREFS

Cf. A033845, A139257.

Sequence in context: A265188 A163903 A074627 * A064957 A008851 A079259

Adjacent sequences:  A067609 A067610 A067611 * A067613 A067614 A067615

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Feb 22 2002

STATUS

approved

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Last modified June 21 06:55 EDT 2021. Contains 345358 sequences. (Running on oeis4.)