Double-Struck Capital Q ℚ

The set of rational numbers ℚ = {m/n} consists of numbers that can be expressed as ordinary fractions (for example, 4/3). In decimal representation, they can be finite or repeating decimals, such as 4/3 = 1.333... The term «rational» comes from the Latin word «quotient», which furthermore developed in English. It's interesting enough that the set ℚ is also countable (equivalent in cardinality to the set of natural numbers ℕ).

The sets ℕ , ℤ , ℚ , ℝ , ℂ are often written in bold font (for example, in the works of Nicolas Bourbaki) or as double-struck letters on blackboards and manuscripts. The usage of double-struck font in printed texts was introduced by American mathematicians and became widespread with the famous textbook on complex analysis by Gunning and Rossi, published in 1965.

Il simbolo «Double-Struck Capital Q» è incluso nel sotto-blocco «Simboli simili a lettere» del blocco «lettere simboli» ed è stato approvato come parte della versione Unicode 1.1 nel 1993.