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A032742
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a(1) = 1; for n > 1, a(n) = largest proper divisor of n.
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121
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1, 1, 1, 2, 1, 3, 1, 4, 3, 5, 1, 6, 1, 7, 5, 8, 1, 9, 1, 10, 7, 11, 1, 12, 5, 13, 9, 14, 1, 15, 1, 16, 11, 17, 7, 18, 1, 19, 13, 20, 1, 21, 1, 22, 15, 23, 1, 24, 7, 25, 17, 26, 1, 27, 11, 28, 19, 29, 1, 30, 1, 31, 21, 32, 13, 33, 1, 34, 23, 35, 1, 36, 1, 37, 25, 38, 11, 39, 1, 40
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OFFSET
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1,4
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COMMENTS
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Seems equal to a(n)=Max{GCD[n,j]; j=n+1,..,2n-1} - Labos Elemer, May 22 2002. This is correct: No integer in range [n+1, 2n-1] has n as its divisor, but certainly at least one multiple of the largest proper divisor of n will occur there [e.g., if it is n/2, then at n + (n/2)]. - Antti Karttunen, Dec 18 2014
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LINKS
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T. D. Noe and Rémi Eismann, Table of n, a(n) for n=1..10000 (first 1000 terms from T. D. Noe)
Eric Weisstein's World of Mathematics, Proper Divisor
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FORMULA
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a(n) = n / A020639(n).
Other identities and observations:
A054576(n) = a(a(n)); A117358(n) = a(a(a(n))) = a(A054576(n)); a(A008578(n)) = 1, a(A002808(n)) > 1. - Reinhard Zumkeller, Mar 10 2006
a(n) = A130064(n) / A006530(n). - Reinhard Zumkeller, May 05 2007
a(m)*a(n) < a(m*n) for m and n > 1. - Reinhard Zumkeller, Apr 11 2008
a(m*n) = max(m*a(n), n*a(m)). - Robert Israel, Dec 18 2014
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MAPLE
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A032742 :=proc(n) option remember; if n = 1 then 1; else numtheory[divisors](n) minus {n} ; max(op(%)) ; end if; end proc: # R. J. Mathar, Jun 13 2011
1, seq(n/min(numtheory:-factorset(n)), n=2..1000); # Robert Israel, Dec 18 2014
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MATHEMATICA
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f[n_] := If[n == 1, 1, Divisors[n][[-2]]]; Table[f[n], {n, 100}] (* Vladimir Joseph Stephan Orlovsky, Mar 03 2010 *)
Join[{1}, Divisors[#][[-2]]&/@Range[2, 80]] (* Harvey P. Dale, Nov 29 2011 *)
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PROG
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(PARI) a(n)=if(n==1, 1, n/factor(n)[1, 1]) \\ Charles R Greathouse IV, Jun 15 2011
(Haskell)
a032742 n = n `div` a020639 n -- Reinhard Zumkeller, Oct 03 2012
(Scheme) (define (A032742 n) (/ n (A020639 n))) ;; Antti Karttunen, Dec 18 2014
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CROSSREFS
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Equals n/A020639(n).
Cf. A032741, A052126, A068319, A063928.
Sequence in context: A247892 A079880 A159353 * A060654 A280497 A280495
Adjacent sequences: A032739 A032740 A032741 * A032743 A032744 A032745
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KEYWORD
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nonn,easy,nice
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AUTHOR
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Patrick De Geest, May 15 1998
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STATUS
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approved
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