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 A292369 Numbers n such that f(f(f(n))) = f(f(n)) + f(n) where f = A000203. 0
 2, 4, 16, 25, 64, 4096, 65536, 262144, 1073741824 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that A066971(n) = A051027(n) + A000203(n). A061652 is a subsequence. Are there any odd terms other than 25? LINKS EXAMPLE 25 = 5^2 is a term because sigma(sigma(sigma(5^2))) = sigma(2^5) = sigma(sigma(5^2)) + sigma(5^2). MATHEMATICA f[n_] := DivisorSigma[1, n]; fQ[n_] := f[f[f[n]]] == f[f[n]] + f[n]; Select[ Range@1000000, fQ] (* Robert G. Wilson v, Sep 23 2017 *) PROG (PARI) s(n) = sigma(n); isok(n) = s(s(s(n)))==s(s(n))+s(n); CROSSREFS Cf. A000203, A019279, A051027, A061652, A066971. Sequence in context: A275674 A225983 A298080 * A153665 A015775 A330582 Adjacent sequences:  A292366 A292367 A292368 * A292370 A292371 A292372 KEYWORD nonn,more AUTHOR Altug Alkan, Sep 15 2017 EXTENSIONS a(9) from Giovanni Resta, Sep 15 2017 STATUS approved

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Last modified January 24 04:35 EST 2020. Contains 331183 sequences. (Running on oeis4.)