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A071395 Primitive abundant numbers (abundant numbers all of whose proper divisors are deficient numbers). 15
20, 70, 88, 104, 272, 304, 368, 464, 550, 572, 650, 748, 836, 945, 1184, 1312, 1376, 1430, 1504, 1575, 1696, 1870, 1888, 1952, 2002, 2090, 2205, 2210, 2470, 2530, 2584, 2990, 3128, 3190, 3230, 3410, 3465, 3496, 3770, 3944, 4030, 4070, 4095, 4216, 4288 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This is a subsequence of the primitive abundant number sequence A091191, since none of these numbers are a positive integer multiple of a perfect number (A000396). - Timothy L. Tiffin, Jul 15 2016

If the terms of this sequence are removed from A091191, then the resulting sequence will be A275082. - Timothy L. Tiffin, Jul 16 2016

Numbers n such that A294927(n) = 0 and A294937(n) = 1. - Antti Karttunen, Nov 14 2017

REFERENCES

Guy, R. K. Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, p. 46, also section B2, 1994.

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Primitive Abundant Number

EXAMPLE

20 is a term since 1, 2, 4, 5, and 10 (the proper divisors of 20) are all deficient numbers. - Timothy L. Tiffin, Jul 15 2016

MAPLE

abundance:= proc(n) option remember;  numtheory:-sigma(n)-2*n end proc:

select(n -> abundance(n) > 0 and andmap(t -> abundance(t) < 0, numtheory:-divisors(n) minus {n}), [$1..10000]); # Robert Israel, Nov 15 2017

MATHEMATICA

Select[Range@ 5000, DivisorSigma[1, #] > 2 # && Times @@ Boole@ Map[DivisorSigma[1, #] < 2 # &, Most@ Divisors@ #] == 1 &] (* Michael De Vlieger, Jul 16 2016 *)

PROG

(PARI) isA071395(v) = {if (sigma(v) <= 2*v, return (0)); fordiv (v, d, if ((d != v) && (sigma(d) >= 2*d), return (0)); ); return (1); } \\ Michel Marcus, Mar 10 2013

CROSSREFS

Cf. A006038, A000396, A005100, A005101, subsequence of A091191, A275082.

Cf. A294927, A294937.

Sequence in context: A238101 A153728 A234367 * A245856 A053741 A303609

Adjacent sequences:  A071392 A071393 A071394 * A071396 A071397 A071398

KEYWORD

nonn

AUTHOR

Joe McCauley (mccauley(AT)davesworld.net), Jun 12 2002

EXTENSIONS

Offset corrected by Donovan Johnson, Aug 28 2011

STATUS

approved

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Last modified October 17 06:27 EDT 2018. Contains 316276 sequences. (Running on oeis4.)