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 A272636 a(0)=0, a(1)=1; thereafter a(n) = squarefree part of a(n-1)+a(n-2). 5
 0, 1, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7, 1, 2, 3, 5, 2, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Periodic with period {1,2,3,5,2,7}. James Propp, in a posting to the Math Fun list, asks if every sequence of positive numbers satisfying the same recurrence will eventually merge with this sequence (as A272638 does). The answer is no, Fred W. Helenius found infinitely many counterexamples, including A272637. See A272639 for other counterexamples which start 1,x. Other counter-examples found by Helenius include [n, 2n, 3n, 5n, 2n, 7n] (period 6) where n is any squarefree positive integer coprime to 210 = 2*3*5*7. LINKS PROG (Python) from sympy.ntheory.factor_ import core l=[0, 1] for n in xrange(2, 101):     l+=[core(l[n - 1] + l[n - 2]), ] print l # Indranil Ghosh, Jun 03 2017 CROSSREFS Cf. A007913 (squarefree part of n), A000045, A272637, A272638, A272639. See A165911 for a similar sequence. See also A214674, A214892-A214898. Sequence in context: A138512 A053723 A201652 * A066949 A073481 A178094 Adjacent sequences:  A272633 A272634 A272635 * A272637 A272638 A272639 KEYWORD nonn AUTHOR N. J. A. Sloane, May 05 2016 STATUS approved

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Last modified June 16 21:13 EDT 2019. Contains 324155 sequences. (Running on oeis4.)