OFFSET
1,4
COMMENTS
If n is odd and k >= 1, then a(2^k*n) = (k+1)*n+k if n is in A000069 and (k+1)*n if n is not in A000069. - Robert Israel, Oct 03 2017
LINKS
FORMULA
EXAMPLE
For n = 55 whose proper divisors are 1, 5 and 11 (in binary "1", "101" and "1011"), only 1 and 11 have an odd number of 1's in their binary representations, thus a(55) = 2.
MAPLE
f:= proc(n) nops(select(t -> convert(convert(t, base, 2), `+`)::odd, numtheory:-divisors(n) minus {n})) end proc:
map(f, [$1..200]); # Robert Israel, Oct 03 2017
MATHEMATICA
Table[DivisorSum[n, 1 &, And[OddQ@ DigitCount[#, 2, 1], # < n] &], {n, 120}] (* Michael De Vlieger, Oct 03 2017 *)
PROG
(PARI) A290090(n) = sumdiv(n, d, (d<n)*(hammingweight(d)%2));
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Antti Karttunen, Oct 03 2017
STATUS
approved