双线大写 N ℕ

The set of natural numbers ℕ = {1, 2, 3, ...} consists of numbers used in counting. Sometimes, 0 is added to this set. Sets that have the same cardinality as ℕ (countable sets) are the 「smallest」 of infinite sets.

Sets ℕ , ℤ , ℚ , ℝ , and ℂ are often written in bold (for example, in the works of Nicolas Bourbaki), and on blackboards and manuscripts, they are written in double letters. American mathematicians introduced double font in printed texts, and it became popular with the famous 1965 textbook on complex analysis by Gunning and Rossi.

符号 「双线大写 N」 包含在 「字母式符号」 块的 「字母符號」 子块中，并在1993年作为Unicode版本1.1的一部分获得批准。