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A231086 Initial members of abundant twins, i.e., values of k such that (k, k+2) are both abundant numbers. 12
18, 40, 54, 70, 78, 88, 100, 102, 112, 138, 160, 174, 196, 198, 208, 220, 222, 258, 270, 280, 304, 306, 318, 340, 348, 350, 352, 364, 366, 378, 390, 400, 414, 438, 448, 460, 462, 474, 490, 498, 520, 532, 544, 550, 558, 570, 580, 606, 616, 618, 640, 642, 648 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..5000 from Shyam Sunder Gupta)

EXAMPLE

18, 20 are abundant, thus the smaller number is listed.

MAPLE

withnumtheory: select(n->sigma(n)>2*n and sigma(n+1)<2*(n+1) and sigma(n+2)>2*(n+2), [$1..700]); # Muniru A Asiru, Jun 24 2018

MATHEMATICA

AbundantQ[n_] := DivisorSigma[1, n] > 2n; m = 0; a2 = {}; Do[If[AbundantQ[n], m = m + 1; If[m > 1, AppendTo[a2, n - 2]], m = 0], {n, 2, 100000, 2}]; a2

Module[{nn=650, sa}, sa=Table[If[DivisorSigma[1, n]>2n, 1, 0], {n, nn}]; Transpose[ SequencePosition[sa, {1, 0, 1}]]][[1]] (* The program uses the SequencePosition function from Mathematica version 10 *) (* Harvey P. Dale, May 20 2016 *)

PROG

(PARI) is(n)=sigma(n, -1)>2 && sigma(n+2, -1)>2 \\ Charles R Greathouse IV, Feb 21 2017

(GAP) A:=Filtered([1..700], n->Sigma(n)>2*n);;  a:=List(Filtered([1..Length(A)-1], i->A[i+1]=A[i]+2), j->A[j]); # Muniru A Asiru, Jun 24 2018

CROSSREFS

Cf. A005101, A108926, A231088, A231089, A231090, A231092, A231093.

Sequence in context: A043118 A039295 A043898 * A285527 A097972 A154284

Adjacent sequences:  A231083 A231084 A231085 * A231087 A231088 A231089

KEYWORD

nonn,changed

AUTHOR

Shyam Sunder Gupta, Nov 03 2013

STATUS

approved

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Last modified January 24 04:30 EST 2020. Contains 331182 sequences. (Running on oeis4.)