Quoted from: A Selection of problems in the Theory of Numbers, by Waclaw Sierpinski, (1964)

page 104, line 13, problem $P_{19}^2$

Do there exist infinitely many twin primes ?

page 120, line 19, problem $P_{96}^2$

Do there exist infinitely many natural numbers which cannot be put in any of the four forms
$6xy \pm x \pm y$ where $x$ and $y$ are natural numbers ?

lines 20 and 21, page 120: 

It can be shown that question $P_{96}^2$ is equivalent to question $P_{19}^2.$