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A071834 Numbers n > 1 such that n and sigma(n) have the same largest prime factor. 3
6, 28, 40, 84, 117, 120, 135, 140, 224, 234, 270, 420, 468, 496, 585, 672, 756, 775, 819, 891, 931, 936, 1080, 1120, 1170, 1287, 1372, 1488, 1550, 1625, 1638, 1782, 1862, 2176, 2299, 2325, 2340, 2480, 2574, 2793, 3100, 3159, 3250, 3276, 3360, 3472 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

By pure convention, we could include a leading 1 to this sequence, as someone using the mathematically arguably value A006530(1) = 1 might search for this sequence with a leading 1. However, this was not done in view of the age of this sequence. - Rémy Sigrist, Jan 09 2018

LINKS

Michel Marcus, Table of n, a(n) for n = 1..1000

FORMULA

n such that A006530(n) = A006530(sigma(n)).

n such that A006530(n) = A071190(n). - Michel Marcus, Oct 11 2017

EXAMPLE

1550 = 2*5^2*31 and sigma(1550) = 2976 = 2^5*3*31 hence 1550 is in the sequence.

MATHEMATICA

fQ[n_] := FactorInteger[n][[-1, 1]] == FactorInteger[DivisorSigma[1, n]][[-1, 1]]; Rest@ Select[ Range@3500, fQ] (* Robert G. Wilson v, Jan 09 2018 *)

PROG

(PARI) for(n=2, 1000, if(component(component(factor(n), 1), omega(n)) == component(component(factor(sigma(n)), 1), omega(sigma(n))), print1(n, ", ")))

(PARI) isok(n) = vecmax(factor(n)[, 1]) == vecmax(factor(sigma(n))[, 1]); \\ Michel Marcus, Sep 29 2017

CROSSREFS

Cf. A000203 (sigma), A006530 (gpf), A071190.

A000396 (perfect numbers) is a subsequence.

Sequence in context: A252234 A242344 A247111 * A295078 A055196 A323752

Adjacent sequences:  A071831 A071832 A071833 * A071835 A071836 A071837

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Jun 08 2002

STATUS

approved

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Last modified September 16 06:38 EDT 2019. Contains 327090 sequences. (Running on oeis4.)