A066481 Largest anti-divisor of n.

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, 20, 21, 21, 22, 23, 23, 24, 25, 25, 26, 27, 27, 28, 29, 29, 30, 31, 31, 32, 33, 33, 34, 35, 35, 36, 37, 37, 38, 39, 39, 40, 41, 41, 42, 43, 43, 44, 45, 45, 46, 47, 47, 48, 49, 49, 50

Offset

3,1

Comments

Apart from initial terms this is identical to A004396.

See A066272 for definition of anti-divisor.

Links

Seiichi Manyama, Table of n, a(n) for n = 3..10000

Maple

antidivisors := proc(n)
   local a, k;
   a := {} ;
   for k from 2 to n-1 do
      if abs((n mod k)- k/2) < 1 then
         a := a union {k} ;
      end if;
   end do:
   a ;
end proc:
A066481 := proc(n)
if n < 3 then
    return 0;
else
    sort(convert(antidivisors(n), list)) ;
    op(-1, %) ;
end if;
end proc: # R. J. Mathar, Mar 15 2013

Mathematica

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} ]

Prog

(PARI) a(n)=2*n\/3 \\ Charles R Greathouse IV, Feb 27 2013

Crossrefs

Cf. A066482.

Sequence in context: A168052 A131737 A004396 * A248103 A121928 A175406

Adjacent sequences: A066478 A066479 A066480 * A066482 A066483 A066484

Keyword

nonn

Author

Robert G. Wilson v, Jan 02 2002

Status

approved