I won't go into great detail as to why, because most of your eyes will bleed out of your head if I do, but suffice to say, a number of that form is fairly easy to test for primacy.
詳しい理由は言いません 説明したら皆さんの 目玉が飛び出すでしょう ただこの形式の数は 素数かどうかを調べるのが とても簡単なのです