$$\left. \begin{array} { l } { a = 4 n - 1 } \\ { a + b | } \\ { a + b | a ^ { n } + b ^ { n } } \end{array} \right.$$
$a=4n-1$
$n\in \mathrm{C}$
$a=4n-1$
$n\in \mathrm{R}$
