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A137210
Numbers k such that abundance(k) = abundance(sigma(k)).
0
1, 84, 252, 12717, 177744, 189264, 143747328
OFFSET
1,2
COMMENTS
a(8) > 3.4*10^10, if it exists. - Amiram Eldar, Apr 06 2024
EXAMPLE
abundance(84) = sigma(84) - 2(84) = 56 = abundance(224) = abundance(sigma(84)), so 84 is a term in the sequence.
MATHEMATICA
abund[n_] := DivisorSigma[1, n]-2n; l = {}; For[i = 1, i <= 10^6, i++, If[abund[i] == abund[DivisorSigma[1, i]], l = Append[l, i]]]; l
PROG
(PARI) is(k) = {my(s = sigma(k)); s - 2*k == sigma(s) - 2*s; } \\ Amiram Eldar, Apr 06 2024
CROSSREFS
Cf. A000203 (sigma), A033880 (abundance).
Sequence in context: A112066 A096383 A220043 * A220009 A160321 A249969
KEYWORD
nonn,more
AUTHOR
Joseph L. Pe, Mar 05 2008
EXTENSIONS
a(7) from Donovan Johnson, Apr 27 2008
STATUS
approved