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URL: https://oeis.org/A292369

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A292369
Numbers n such that f(f(f(n))) = f(f(n)) + f(n) where f = A000203.
1
2, 4, 16, 25, 64, 4096, 65536, 262144, 1073741824
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?
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
KEYWORD
nonn,more
AUTHOR
Altug Alkan, Sep 15 2017
EXTENSIONS
a(9) from Giovanni Resta, Sep 15 2017
STATUS
approved