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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
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: A242344 A344588 A247111 * A295078 A055196 A323752
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Jun 08 2002
STATUS
approved

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Last modified March 29 04:23 EDT 2024. Contains 371264 sequences. (Running on oeis4.)