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 A064814 Greatest common divisor of n and the n-th composite number. 7
 1, 2, 1, 1, 5, 6, 7, 1, 1, 2, 1, 3, 1, 2, 5, 2, 1, 2, 1, 4, 3, 2, 1, 12, 1, 13, 1, 14, 1, 15, 1, 16, 1, 2, 1, 4, 1, 1, 1, 1, 1, 6, 1, 1, 1, 1, 1, 4, 1, 10, 3, 2, 1, 2, 11, 2, 1, 1, 1, 12, 1, 2, 3, 8, 5, 1, 1, 1, 1, 5, 1, 2, 1, 2, 3, 4, 7, 2, 1, 10, 3, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 31, 1, 1, 32, 1, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(12) = gcd(12, A002808(12)) = gcd(12, 21) = gcd(2*2*3, 3*7) = 3. LINKS R. Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA a(n) = gcd(n, A002808(n)). MATHEMATICA Composite[n_Integer] := FixedPoint[n + PrimePi[#] + 1 &, n + PrimePi[n] + 1]; Table[GCD[n, Composite[n]], {n, 100}] (* T. D. Noe, Feb 22 2012 *) Module[{nn=150, comps}, comps=Complement[Range[2, nn], Prime[Range[ PrimePi[ nn]]]]; GCD@@@Thread[{comps, Range[Length[comps]]}]] (* Harvey P. Dale, Apr 23 2014 *) PROG (PARI) nextComp(n)= { if (!isprime(n), return(n)); return(n + 1) } { c=3; for (n=1, 1000, c=nextComp(c + 1); write("b064814.txt", n, " ", gcd(n, c)) ) } \\ Harry J. Smith, Sep 27 2009 CROSSREFS Cf. A002808, A071224. Sequence in context: A210664 A176093 A092437 * A051012 A064644 A306444 Adjacent sequences:  A064811 A064812 A064813 * A064815 A064816 A064817 KEYWORD nice,nonn AUTHOR Reinhard Zumkeller, Oct 22 2001 STATUS approved

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Last modified March 21 01:18 EDT 2019. Contains 321356 sequences. (Running on oeis4.)