Book:Richard K. Guy/Unsolved Problems in Number Theory/Second Edition/Errata

Prime to Own Power minus 1 over Prime minus 1 being Prime
$\mathbf A$: Prime Numbers: $\mathbf {A 3}$: Mersenne primes. Repunits. Fermat numbers. Primes of shape $k \cdot 2^n + 1$.

Numbers $n$ whose Euler Phi value Divides $n + 1$
$\mathbf B$: Divisibility: $\mathbf {B 37}$: Does $\map \phi n$ properly divide $n - 1$?

Sums of Squares
$\mathbf C$: Additive Number Theory: $\mathbf {C 20}$: Sums of Squares