%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 antidivisor of n.
%C Apart from initial terms this is identical to A004396.
%C See A066272 for definition of antidivisor.
%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 n1 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
