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 A291881 Numbers n such that sigma(sigma(n)) = sigma(sigma(n)-n) + sigma(n); that is, f(g(n)) = g(f(n)) where f = A000203 and g = A001065. 1
 2, 38040, 51888, 236644, 260880, 3097024, 5283852, 5667312, 11777472, 46120848, 52981252, 69128640, 121352208, 330364848, 485906400, 662736600, 769422720, 1111869360, 1267978320, 1272335760, 1426817904, 1807128528, 2107406448, 2381691312, 2452404544, 2691587568 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Initial motivation for this sequence was that question: Can be an odd number k such that f(g(k)) = g(f(k)) where f = A000203 and g = A001065? Non-abundant terms are 2, 236644, 52981252,... If an odd term exists, it is larger than 2*10^11. - Giovanni Resta, Sep 15 2017 LINKS EXAMPLE 38040 is a term because sigma(38040) = 114480 and sigma(114480) = sigma(76440) + 114480. MATHEMATICA inQ[n_] :=  DivisorSigma[1, DivisorSigma[1, n]] == DivisorSigma[1, DivisorSigma[1, n] - n] + DivisorSigma[1, n]; (* Robert G. Wilson v, Sep 10 2017 *) PROG (PARI) a001065(n) = sigma(n)-n; isok(n) = sigma(a001065(n))==a001065(sigma(n)); CROSSREFS Cf. A000203, A001065, A051027, A072869. Sequence in context: A206854 A330304 A272166 * A257968 A303738 A055578 Adjacent sequences:  A291878 A291879 A291880 * A291882 A291883 A291884 KEYWORD nonn AUTHOR Altug Alkan, Sep 05 2017 STATUS approved

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Last modified October 28 19:06 EDT 2020. Contains 338064 sequences. (Running on oeis4.)