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A257720
Down-up reversals: if n is deficient and s(n) is nondeficient, where s(n) is the sum of proper divisors of n.
3
25, 34, 44, 74, 81, 106, 121, 124, 134, 184, 194, 202, 218, 268, 274, 284, 289, 314, 346, 361, 386, 394, 441, 514, 524, 529, 538, 554, 590, 604, 625, 634, 652, 674, 694, 698, 716, 724, 729, 752, 764, 778, 790, 794, 824, 841, 844, 884, 914, 922, 950, 974, 988
OFFSET
1,1
COMMENTS
Larger member of amicable pairs (A002046) belong to this sequence.
LINKS
P. Pollack and C. Pomerance, Some problems of Erdos on the sum-of-divisors function, 2015.
EXAMPLE
284 is deficient and its sum of proper divisors is 220 which in turn is nondeficient. Hence 284 is in the sequence.
MATHEMATICA
spd[n_]:=DivisorSigma[1, n]-n; Select[Range[988], 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)
a257720 n = a257720_list !! (n-1)
a257720_list = filter f [1..] where
f x = sx > 0 && 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. A257719 (up-down reversals).
Sequence in context: A227516 A281369 A188059 * A098368 A281370 A101066
KEYWORD
nonn
AUTHOR
Michel Marcus, May 05 2015
STATUS
approved