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 A289126 Numbers n such that usigma(usigma(n))/usigma(n) > usigma(usigma(m))/usigma(m) for all m < n, where usigma(n) is the sum of unitary divisors of n (A034448). 1
 1, 2, 5, 18, 164, 1538, 20018, 408404, 7759748, 148728578, 6976017044, 7317446132 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The unitary version of A289124. LINKS Paul ErdÅ‘s and M. V. Subbarao, On the iterates of some arithmetic functions, The theory of arithmetic functions (Proc. Conf., Western Michigan Univ., Kalamazoo, Mich. 1971), Lecture Notes in Math., 251 , pp. 119-125, Springer, Berlin, 1972. [alternate link] MATHEMATICA usigma[n_] := If[n == 1, 1, Times @@ (1 + Power @@@ FactorInteger[n])]; a = {}; k=1; rmax = 0; While[Length[a]<8, s = usigma[ k]; s2 = usigma[ s]; r = s2/s;  If[r > rmax, AppendTo[a, k]; rmax = r]; k++]; a CROSSREFS Cf. A034448, A055033, A289124. Sequence in context: A281532 A097584 A197855 * A062097 A323706 A125765 Adjacent sequences:  A289123 A289124 A289125 * A289127 A289128 A289129 KEYWORD nonn,more AUTHOR Amiram Eldar, Jun 25 2017 EXTENSIONS a(10)-a(12) from Giovanni Resta, Aug 23 2017 STATUS approved

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Last modified July 30 23:12 EDT 2021. Contains 346365 sequences. (Running on oeis4.)