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 A173430 Last of consecutive coprime iterations of sum-of-divisors function 2
 1, 15, 15, 15, 6, 6, 15, 15, 14, 10, 12, 12, 14, 14, 15, 104, 18, 18, 20, 20, 104, 22, 24, 24, 104, 26, 40, 28, 30, 30, 104, 104, 33, 34, 48, 91, 38, 38, 56, 40, 42, 42, 44, 44, 45, 46, 48, 48, 80, 255, 51, 52, 54, 54, 72, 56, 80, 58, 60, 60, 62, 62, 104, 255, 84, 66, 68, 68 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES Oystein Ore, Number Theory and Its History, 1988, Dover Publications, ISBN 0486656209, pp. 88-96. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 Graeme L. Cohen and Herman J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, Vol. 5, No. 2 (1996), pp. 91-100. Leonard Eugene Dickson, History of the Theory of Numbers, Volume I, Divisibility and Primality, Carnegie Institution of Washington, 1919, Chapters II and X EXAMPLE Calculating sum-of-divisors ( ... sum-of-divisors ( sum-of-divisors ( 4 ) ) ... ) the iterates are 4, 7, 8, 15, 24, ... . The initial, consecutive, pairwise, coprime iterates are 4, 7, 8, 15, so a(4) = 15 . Here sigma ( 4 ) = 7, sigma ( sigma ( 4 ) ) = sigma ( 7 ) = 8, etc. MATHEMATICA a[1] = 1; a[n_] := Module[{k = n}, While[CoprimeQ[k, (s = DivisorSigma[1, k])], k = s]; k]; Array[a, 68] (* Amiram Eldar, Sep 02 2019 *) CROSSREFS Cf. A129246 and the references there, A019294, A019295, A000203, A051027, A019284, A019277. Sequence in context: A069785 A290850 A290669 * A010854 A003884 A225917 Adjacent sequences:  A173427 A173428 A173429 * A173431 A173432 A173433 KEYWORD easy,nonn AUTHOR Walter Nissen, Feb 18 2010 STATUS approved

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Last modified February 20 17:04 EST 2020. Contains 332080 sequences. (Running on oeis4.)