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A265726 Primitive weird numbers whose abundance is a record. 2
70, 836, 7192, 9272, 73616, 243892, 338572, 1188256, 1901728, 3963968, 28279232, 36228736, 91322752, 141659096, 263144192, 351295232, 664373504, 2113834496, 5522263024, 6933503488, 19179527168, 22755515392, 31574500724, 98620009472, 135895635968 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Although the abundance A(n) = sigma(n) - 2n is increasing, the (relative) abundancy sigma(n)/n is decreasing, except at indices {3, 6, 8, 15, 16, 19, 24 ...}. No term has larger abundancy than 2 + 2/35, that of a(1). - M. F. Hasler, Nov 14 2018

LINKS

Table of n, a(n) for n=1..25.

Douglas E. Iannucci, On primitive weird numbers of the form 2^k*p*q, arXiv:1504.02761 [math.NT], 2015.

Giuseppe Melfi, On the conditional infiniteness of primitive weird numbers, Journal of Number Theory, Vol. 147, Feb 2015, pgs 508-514.

Wikipedia, Weird number

EXAMPLE

a(1) = 70 since it is the first term in A002975; its abundance is 4.

a(2) is 836 since its abundance, 8, exceeds that of a(1); 4.

a(3) is 7192 = A002975(5) since its abundance, 16, exceeds that of a(2) and that of A002975(1..4).

MATHEMATICA

(* copy the terms from A002975, assign them equal to 'lst' and then *) f[n_] := DivisorSigma[1, n] - 2n; k = 1; lsu = {}; mx = 0; While[k < 647, ds = f@ lst[[k]]; If[ds > mx, mx = ds; AppendTo[lsu, lst[[k]]]]; k++]; lsu

CROSSREFS

Cf. A002975, A258250, A258333, A258374, A258375, A258401, A258882, A258883, A258884, A258885, A265727, A265728.

Sequence in context: A329190 A258250 A258882 * A258375 A306953 A302573

Adjacent sequences:  A265723 A265724 A265725 * A265727 A265728 A265729

KEYWORD

nonn

AUTHOR

Douglas E. Iannucci and Robert G. Wilson v, Dec 14 2015

STATUS

approved

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Last modified February 24 01:16 EST 2020. Contains 332195 sequences. (Running on oeis4.)