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 A263837 Non-abundant numbers (or nonabundant numbers): complement of A005101; numbers k for which sigma(k) <= 2*k. 18
 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 17, 19, 21, 22, 23, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 37, 38, 39, 41, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 55, 57, 58, 59, 61, 62, 63, 64, 65, 67, 68, 69, 71, 73, 74, 75, 76, 77, 79, 81, 82, 83, 85, 86, 87, 89, 91, 92, 93, 94, 95, 97, 98, 99 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For all n, A003961(a(n)) is in A005100. - Antti Karttunen, Aug 28 2020 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Paul Pollack and Carl Pomerance, Some problems of Erdős on the sum-of-divisors function, For Richard Guy on his 99th birthday: May his sequence be unbounded, Trans. Amer. Math. Soc. Ser. B 3 (2016), 1-26. FORMULA A001065(a(n)) <= a(n). - Reinhard Zumkeller, Oct 31 2015 MAPLE isA263837 := proc(n) if 2*n-numtheory[sigma](n) >=0 then true; else false; end if; end proc: A263837 := proc(n) option remember; local a; if n =1 then 1; else for a from procname(n-1)+1 do if isA263837(a) then return a; end if; end do: end if; end proc: seq(A263837(n), n=1..100) ; # R. J. Mathar, Jun 06 2024 MATHEMATICA Select[Range[100], DivisorSigma[1, #] <= 2*# &] (* Amiram Eldar, Mar 14 2024 *) PROG (Haskell) a263837 n = a263837_list !! (n-1) a263837_list = filter (\x -> a001065 x <= x) [1..] -- Reinhard Zumkeller, Oct 31 2015 (PARI) isok(n) = sigma(n) <= 2*n; \\ Michel Marcus, Dec 27 2015 CROSSREFS Union of A000396 and A005100. Cf. A005101 (complement), A023196, A294935 (characteristic function). Cf. A000203, A001065, A003961. Sequence in context: A368007 A360553 A067340 * A283550 A271113 A325369 Adjacent sequences: A263834 A263835 A263836 * A263838 A263839 A263840 KEYWORD nonn AUTHOR N. J. A. Sloane, Oct 30 2015 EXTENSIONS Additional description to the definition added by Antti Karttunen, Aug 28 2020 STATUS approved

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Last modified July 24 22:37 EDT 2024. Contains 374585 sequences. (Running on oeis4.)