On Nowak numbers

Nowak numbers are the integer $n$ such that $2^n+1$ is divisible by $n$. The first several terms of this sequence are $1$, $3$, $9$, $27$, $81$, $171$, $243$, $513$, $729$, $1539...$ In the talk, we will discuss new lower bounds for the quantity of Nowak numbers $n$ whics are less or equal to $x$. Also, we will consider some generalizations of these sequence.