Spectra and systems of equations

In a previous work we introduced an elementary method to analyze the periodicity of a generating function defined by a single equation y=G(x,y). This was based on deriving a single set-equation Y = Gammma(Y) defining the spectrum of the generating function. This paper focuses on extending the analysis of periodicity to generating functions defined by a system of equations y = G(x,y). The final section looks at periodicity results for the spectra of monadic second-order classes whose spectrum is determined by an equational specification - an observation of Compton shows that monadic-second order classes of trees have this property. This section concludes with a substantial simplification of the proofs in the 2003 foundational paper on spectra by Gurevich and Shelah, namely new proofs are given of: (1) every monadic second-order class of $m$-colored functional digraphs is eventually periodic, and (2) the monadic second-order theory of finite trees is decidable.