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A140798 Harmonic numbers that are not multiply-perfect. 1
140, 270, 1638, 2970, 6200, 8190, 18600, 18620, 27846, 55860, 105664, 117800, 167400, 173600, 237510, 242060, 332640, 360360, 539400, 695520, 726180, 753480, 950976, 1089270, 1421280, 1539720, 2229500, 2290260, 2457000, 2845800, 4358600 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Sequence is A001599 excluding those entries that appear in A007691.

Multiply-perfect numbers m (with sigma(m)/m an integer) are necessarily harmonic numbers (with tau(m)/{sigma(m)/m } an integer), but the converse is not true : If m divides sigma(m), then quotient sigma(m)/m divides tau(m) [m=A007691]; However, quotient tau(n)/{sigma(n)/n} being an integer does not imply quotient sigma(n)/n is necessarily an integer [n=A001599].

REFERENCES

J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 140, pp 48, Ellipses, Paris 2008.

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..912 (terms < 10^14)

MATHEMATICA

Select[Range[10^6], (d = Divisors[#]; IntegerQ[HarmonicMean[d]] && Mod[Total[d], #] != 0)&] (* Jean-Fran├žois Alcover, Nov 13 2017 *)

CROSSREFS

Cf. A000203, A054030, A000005.

Sequence in context: A276026 A259718 A090945 * A249554 A196447 A255783

Adjacent sequences:  A140795 A140796 A140797 * A140799 A140800 A140801

KEYWORD

nonn

AUTHOR

Lekraj Beedassy, Jul 15 2008

EXTENSIONS

Initial term 1 removed and offset corrected by Donovan Johnson, Jan 25 2012

STATUS

approved

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Last modified October 16 10:09 EDT 2018. Contains 316262 sequences. (Running on oeis4.)