🔖 Sierpinski number | Wikipedia

Bookmarked Sierpiński number (Wikipedia)

In number theory, a Sierpinski or Sierpiński number is an odd natural number k such that {\displaystyle k\times 2^{n}+1} is composite, for all natural numbers n. In 1960, Wacław Sierpiński proved that there are infinitely many odd integers k which have this property. In other words, when k is a Sierpiński number, all members of the following set are composite:

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