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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[#]]<spd[#]&] (* Ivan N. Ianakiev, May 06 2015 *)

PROG

(PARI) isok(n) = (sn = sigma(n)-n) && (sn >= 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.)