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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Fletcher, Peter; Hoyle, Hughes; Patty, C. Wayne (1991). Foundations of Discrete Mathematics. PWS-Kent. p. 114. ISBN 0-53492-373-9. An alphabet is a nonempty finite set the members of which are called symbols or characters.

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.

Rosen, Kenneth H. (2012). Discrete Mathematics and Its Applications (PDF) (7th ed.). New York: McGraw Hill. pp. 847–851. ISBN 978-0-07-338309-5. A vocabulary (or alphabet) V is a finite, nonempty set of elements called symbols. A word (or sentence) over V is a string of finite length of elements of V.

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.