OFFSET
1,1
COMMENTS
All terms of this sequence are even.
A139711 contains all positive integers not in this sequence and vice versa.
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
The divisors of 12 are 1,2,3,4,6,12. The middle 2 divisors are 3 and 4. The sum of these is 7, which is odd. So 12 is included in the sequence.
MAPLE
A033676 := proc(n) local d ; for d from floor(sqrt(n)) to 1 by -1 do if n mod d = 0 then RETURN(d) ; fi ; od: end: A033677 := proc(n) n/A033676(n) ; end: isA139710 := proc(n) RETURN ( ( A033676(n)+A033677(n) ) mod 2 = 1 ) ; end: for n from 1 to 300 do if isA139710(n) then printf("%d, ", n) ; fi ; od: # R. J. Mathar, May 11 2008
MATHEMATICA
centralDivisors:=#[[({Floor[#], Ceiling[#]}&[(1+#)/2&[Length[#]]])]]&[Divisors[#]]&;
Select[Range[500], OddQ[Total[#]]&[centralDivisors[#]]&](* Peter J. C. Moses, May 31 2019 *)
Select[Range[158], !IntegerQ[Median[Divisors[#]]] &] (* Stefano Spezia, Mar 14 2023 *)
PROG
(PARI) b(n) = {local(d); d=divisors(n); d[(length(d)+1)\2] + d[length(d)\2+1]};
for(n=1, 180, if(b(n)%2==1, print1(n, ", ")) ) \\ G. C. Greubel, May 31 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Apr 30 2008
EXTENSIONS
More terms from R. J. Mathar, May 11 2008
STATUS
approved