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 A153501 Abundant numbers n such that n/(sigma(n)-2n) is an integer. 11
 12, 18, 20, 24, 40, 56, 88, 104, 120, 196, 224, 234, 368, 464, 650, 672, 992, 1504, 1888, 1952, 3724, 5624, 9112, 11096, 13736, 15376, 15872, 16256, 17816, 24448, 28544, 30592, 32128, 77744, 98048, 122624, 128768, 130304, 174592, 396896, 507392 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Sigma(n)-2n is the abundance of n. The only odd term in this sequence < 2*10^12 is 173369889. - Donovan Johnson, Feb 15 2012 Equivalently, the abundancy of n, ab=sigma(n)/n, satisfies the following relation: numerator(ab) = 2*denominator(ab)+1, that is, ab=(2k+1)/k where k is the integer ratio mentioned in definition. - Michel Marcus, Nov 07 2014 The tri-perfect numbers (A005820) are in this sequence, since their abundancy is 3n/n = 3 = (2k+1)/k with k=1. - Michel Marcus, Nov 07 2014 LINKS Donovan Johnson, Table of n, a(n) for n = 1..200 EXAMPLE The abundance of 174592 = sigma(174592)-2*174592 = 43648. 174592/43648 = 4. MAPLE filter:= proc(n) local s; s:= numtheory:-sigma(n); (s > 2*n) and (n mod (s-2*n) = 0) end proc: select(filter, [\$1..10^5]); # Robert Israel, Nov 07 2014 PROG (PARI) isok(n) = ((ab = (sigma(n)-2*n))>0) && (n % ab == 0) \\ Michel Marcus, Jul 16 2013 (Sage) def A153501_list(len): def is_A153501(n): t = sigma(n, 1) - 2*n return t > 0 and t.divides(n) return filter(is_A153501, range(1, len)) A153501_list(1000) # Peter Luschny, Nov 07 2014 CROSSREFS Intersection of A097498 and A005101. Disjoint union of A181595 and A005820. Cf. A000203, A033880. Sequence in context: A290141 A087245 A342100 * A215012 A181595 A263189 Adjacent sequences: A153498 A153499 A153500 * A153502 A153503 A153504 KEYWORD nonn AUTHOR Donovan Johnson, Jan 02 2009 STATUS approved

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Last modified December 10 01:39 EST 2022. Contains 358711 sequences. (Running on oeis4.)