If we take "n" to be a negative integer, then the number "k"2"n" + 1 becomes formula_13. When "k" is odd, this is a fraction in reduced form, with numerator 2|"n"| + "k". A dual Sierpinski number is defined as an odd natural number "k" such that is composite for all natural numbers "n". There is a conjecture that the set of these numbers is the same as the set of Sierpinski numbers; for example, is composite for all natural numbers "n".