van der Poorten, A. J. (1981). "Substitution automata, functional equations and "functions algebraic over a finite field"". Papers in algebra, analysis and statistics (Hobart, 1981). Contemporary Mathematics. Vol. 9. Providence, Rhode Island: American Mathematical Society. pp. 307–312. MR0655988. See in particular p. 308.
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van der Poorten, A. J. (1981). "Substitution automata, functional equations and "functions algebraic over a finite field"". Papers in algebra, analysis and statistics (Hobart, 1981). Contemporary Mathematics. Vol. 9. Providence, Rhode Island: American Mathematical Society. pp. 307–312. MR0655988. See in particular p. 308.