Alphabet (formal languages) (English Wikipedia)

Analysis of information sources in references of the Wikipedia article "Alphabet (formal languages)" in English language version.

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springer.com

Ebbinghaus, H.-D.; Flum, J.; Thomas, W. (1994). Mathematical Logic (2nd ed.). New York: Springer. p. 11. ISBN 0-387-94258-0. By an alphabet ${\mathcal {A}}$ we mean a nonempty set of symbols.

Rautenberg, Wolfgang (2010). A Concise Introduction to Mathematical Logic (PDF) (Third ed.). Springer. p. xx. ISBN 978-1-4419-1220-6. If 𝗔 is an alphabet, i.e., if the elements 𝐬 ∈ 𝗔 are symbols or at least named symbols, then the sequence (𝐬₁,...,𝐬_n)∈𝗔ⁿ is written as 𝐬₁···𝐬_n and called a string or a word over 𝗔.