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 A066481 Largest anti-divisor of n. 4

%I

%S 2,3,3,4,5,5,6,7,7,8,9,9,10,11,11,12,13,13,14,15,15,16,17,17,18,19,19,

%T 20,21,21,22,23,23,24,25,25,26,27,27,28,29,29,30,31,31,32,33,33,34,35,

%U 35,36,37,37,38,39,39,40,41,41,42,43,43,44,45,45,46,47,47,48,49,49,50

%N Largest anti-divisor of n.

%C Apart from initial terms this is identical to A004396.

%C See A066272 for definition of anti-divisor.

%H Seiichi Manyama, <a href="/A066481/b066481.txt">Table of n, a(n) for n = 3..10000</a>

%p antidivisors := proc(n)

%p local a, k;

%p a := {} ;

%p for k from 2 to n-1 do

%p if abs((n mod k)- k/2) < 1 then

%p a := a union {k} ;

%p end if;

%p end do:

%p a ;

%p end proc:

%p A066481 := proc(n)

%p if n < 3 then

%p return 0;

%p else

%p sort(convert(antidivisors(n),list)) ;

%p op(-1,%) ;

%p end if;

%p end proc: # _R. J. Mathar_, Mar 15 2013

%t antid[n_] := Select[ Union[ Join[ Select[ Divisors[2n - 1], OddQ[ # ] && # != 1 &], Select[ Divisors[2n + 1], OddQ[ # ] && # != 1 &], 2n/Select[ Divisors[2*n], OddQ[ # ] && # != 1 &]]], # < n & ]; Table[ Last[ antid[n]], {n, 3, 100} ]

%o (PARI) a(n)=2*n\/3 \\ _Charles R Greathouse IV_, Feb 27 2013

%Y Cf. A066482.

%K nonn

%O 3,1

%A _Robert G. Wilson v_, Jan 02 2002

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Last modified December 15 22:02 EST 2019. Contains 330012 sequences. (Running on oeis4.)