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A066481
Largest anti-divisor of n.
4
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
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
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Jan 02 2002
STATUS
approved