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 A064491 a(1) = 1, a(n+1) = a(n) + tau(a(n)), where tau(n) (A000005) is the number of divisors of n. 20
 1, 2, 4, 7, 9, 12, 18, 24, 32, 38, 42, 50, 56, 64, 71, 73, 75, 81, 86, 90, 102, 110, 118, 122, 126, 138, 146, 150, 162, 172, 178, 182, 190, 198, 210, 226, 230, 238, 246, 254, 258, 266, 274, 278, 282, 290, 298, 302, 306, 318, 326, 330, 346, 350, 362, 366, 374 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) = partial sums of A165930(n). [Jaroslav Krizek, Sep 30 2009] REFERENCES Claudia Spiro, Problem proposed at West Coast Number Theory Meeting, 1977. [If you change the starting term, does the resulting sequence always join this one? Does the parity of terms change infinitely often?] - From N. J. A. Sloane, Jan 11 2013 LINKS T. D. Noe, Table of n, a(n) for n=1..1000 FORMULA It seems likely that there exist constants c_1 and c_2 such that c_1*n*log(n) < a(n) < c_2*n*log(n) for all sufficiently large n. - Franklin T. Adams-Watters, Jun 25 2008 a(n+1) = A062249(a(n)). - Reinhard Zumkeller, Mar 29 2014 MATHEMATICA a[n_] := a[n] = a[n - 1] + DivisorSigma[0, a[n - 1]]; a = 1; Table[a[n], {n, 1, 57}] (* Jean-François Alcover, Oct 11 2012 *) NestList[#+DivisorSigma[0, #]&, 1, 60] (* Harvey P. Dale, Feb 05 2017 *) PROG (PARI) { for (n=1, 1000, if (n>1, a+=numdiv(a), a=1); write("b064491.txt", n, " ", a) ) } \\ Harry J. Smith, Sep 16 2009 (Haskell) a064491 n = a064491_list !! (n-1) a064491_list = iterate a062249 1  -- Reinhard Zumkeller, Mar 29 2014 CROSSREFS Cf. A000005, A165494-A165499. Sequence in context: A064550 A186357 A212988 * A120878 A137996 A080057 Adjacent sequences:  A064488 A064489 A064490 * A064492 A064493 A064494 KEYWORD nice,nonn AUTHOR Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Oct 04 2001 EXTENSIONS Beginning of sequence corrected by T. D. Noe, Sep 13 2007 STATUS approved

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Last modified April 23 05:34 EDT 2021. Contains 343199 sequences. (Running on oeis4.)