Doppeltes getroffenes Kapital 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.

Das Symbol „Doppeltes getroffenes Kapital Q“ ist im „Buchstabenartige Symbole“ Subblock des „Buchstabenähnliche Symbole“ blocks enthalten und wurde als Teil der Unicode-Version 1.1 im Jahr 1993 genehmigt.