*(Wikipedia)*

In number theory, aSierpinskiorSierpiński numberis an odd natural numberksuch that {\displaystyle k\times 2^{n}+1} is composite, for all natural numbersn. In 1960, Wacław Sierpiński proved that there are infinitely many odd integerskwhich have this property. In other words, whenkis a Sierpiński number, all members of the following set are composite:

- {\displaystyle \left\{\,k\cdot {}2^{n}+1:n\in \mathbb {N} \,\right\}.}