OFFSET
1,1
COMMENTS
Larger member of amicable pairs (A002046) 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
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. A257719 (up-down reversals).
KEYWORD
nonn
AUTHOR
Michel Marcus, May 05 2015
STATUS
approved