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 A257719 Up-down reversals: nondeficient n with deficient sum of proper divisors of n. 3
 12, 18, 20, 36, 40, 48, 56, 70, 72, 80, 88, 100, 104, 108, 112, 144, 156, 160, 162, 192, 196, 200, 208, 220, 228, 260, 272, 288, 300, 304, 320, 324, 336, 350, 352, 368, 372, 392, 400, 416, 432, 448, 450, 460, 464, 468, 490, 500, 516, 544, 550, 572, 576, 608 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Lesser member of amicable pairs (A002025) belong to this sequence. LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 P. Pollack and C. Pomerance, Some problems of Erdos on the sum-of-divisors function, 2015. EXAMPLE 220 is abundant (nondeficient) and its sum of proper divisors is 284 which in turn is deficient. Hence 220 is in the sequence. MATHEMATICA spd[n_]:=DivisorSigma[1, n]-n; Select[Range[608], spd[#]>=#&&spd[spd[#]]= n) && (sigma(sn) < 2*sn); (Haskell) a257719 n = a257719_list !! (n-1) a257719_list = filter f [1..] where    f x = sx >= x && a001065 sx < sx where sx = a001065 x -- Reinhard Zumkeller, Oct 31 2015 CROSSREFS Cf. A000396 (perfect), A005100 (deficient), A005101 (abundant). Cf. A000203 (sum of divisors), A001065 (sum of proper divisors). Cf. A257720 (down-up reversals). Cf. A001065, A263838. Sequence in context: A181487 A091191 A259980 * A192819 A071927 A247625 Adjacent sequences:  A257716 A257717 A257718 * A257720 A257721 A257722 KEYWORD nonn AUTHOR Michel Marcus, May 05 2015 STATUS approved

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Last modified April 26 09:52 EDT 2019. Contains 322472 sequences. (Running on oeis4.)