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A330245 Numbers m with a unique subset of the divisors of m that sums to m (A064771) such that sigma(m)/m > sigma(k)/k for all smaller terms k < m of A064771, where sigma(m) is the sum of divisors of m (A000203). 0
6, 20, 78, 1014, 3774, 9514254 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Paul Erdős asked whether there are extra-weird numbers n, i.e., numbers n for which sigma(n)/n > 3, but n is not the sum of a subset of its divisors in two ways. Such numbers, if they exist, are in the intersection of A064771 and A068403, and the least of them is a term of this sequence.

a(6) > 2*10^5.

10^11 < a(7) <= 105590246974194. - Giovanni Resta, Jan 14 2020

REFERENCES

Richard K. Guy, Unsolved Problems in Number Theory, 3rd Edition, Springer, 2004, Section B2, p. 77.

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

The abundancy indices of the terms are sigma(a(n))/a(n) = 2 < 2.1 < 2.153... < 2.165... < 2.174... < 2.1757...

MATHEMATICA

okQ[n_] := Module[{d = Most[Divisors[n]]}, SeriesCoefficient[Series[ Product[ 1+x^i, {i, d}], {x, 0, n}], n] == 1]; seq = {}; rm = 0; Do[If[(r = DivisorSigma[1, n]/n) > rm && okQ[n], rm = r; AppendTo[seq, n]], {n, 1, 4000}]; seq (* after Harvey P. Dale at A064771 *)

CROSSREFS

Cf. A000203, A064771, A068403.

Sequence in context: A240043 A058494 A147979 * A118265 A204271 A255469

Adjacent sequences:  A330242 A330243 A330244 * A330246 A330247 A330248

KEYWORD

nonn,more

AUTHOR

Amiram Eldar, Dec 06 2019

EXTENSIONS

a(6) from Giovanni Resta, Jan 14 2020

STATUS

approved

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Last modified August 19 22:59 EDT 2022. Contains 356231 sequences. (Running on oeis4.)