ゴールドバッハ予想。

Goldbach's Conjecture
In 1742, Christian Goldbach, a German amateur mathematician, sent a letter to Leonhard Euler in which he made the following conjecture:

(A) Every number greater than 2 can be written as the sum of three prime numbers.

Goldbach was considering 1 as a primer number, a convention that is no longer followed. Later on, Euler re-expressed the conjecture as:

(B) Every even number greater than or equal to 4 can be expressed as the sum of two prime numbers.

そんな経緯があったんですか。1が素数に含まれるとな！
素数の定義を変えてみても、ちゃんと必要十分条件になってるのかな。
証明、(B)⇒(A)は簡単だけど(A)⇒(B)がちょっとすぐには出てこない…。