Skip to main content
5 of 6
added 6 characters in body
Greg Martin
  • 12.8k
  • 1
  • 48
  • 72

Most certainly such $k$ does not exist. I am not sure whether there is a chance to prove this unconditionally, as even the situation with Sophie Germain primes (which are primes $p$ such that $2p+1$ is also prime, related to the case $k=2$ of your question) is unclear.

Conditionally, the non-existence of $k$ is an immediate consequence of the Dickson's conjecture stating that for a finite set of linear polynomials $f_1(n)=a_1n+b_1,\dotsc,f_m(n)=a_mn + b_m$ with integer coefficients and $a_1,\dotsc,a_m\ge 1$, there are infinitely many positive integers $n$ for which the values of these polynomials are all prime, unless the product $f_1(n)\dotsb f_m(n)$ has a fixed prime factor. For your problem, take $m=2$, $f_1(n)=n$, and $f_2(n)=kn+(k-1)$.

Seva
  • 23k
  • 2
  • 59
  • 141