# Formal Power Series -- what's in it?

I have the following statement in a paper:

Let $$\Psi$$ be the formal power series defined over the alphabet $$\Omega$$ and the log semiring by: $$(\Psi, (a, b)) = -log(c((a,b)))$$ for $$(a,b) \in \Omega$$, and let $$S$$ be the formal pwer series $$S$$ over the log semiring defined by: $$S=\Omega^*+\Psi+\Omega^*$$ (an alphabet is a finite set of symbols and $$\Omega$$ contains pairs of such symbols.)

$$S$$ is a rational power series as a +-product and closure of the polynomial power series $$\Omega$$ and $$\Psi$$.

What exactly is meant here? I know about the automata theoretical aspects, but I haven't heard of the notion "formal power series" over an alphabet and a semiring. How can I think of this?

The paper is at http://www.cs.nyu.edu/~mohri/pub/, [99], page 15 and page 17.

• Is that a verbatim copy from the paper? Jan 24, 2011 at 5:02
• Could you provide a link or a precise reference to the paper, please?
– t.b.
Jan 24, 2011 at 5:04
• yes. updated. thanks. Jan 24, 2011 at 5:17

The idea comes from group there where you have the operations of multiplication and addition in a ring. you can form the expressions $ax + bx^2 + cx^3$, etc... e.g., the polynomials or P[x] but note x may not necessarily be in the ring. We can still write such expressions and take x as a sort of place holder or symbol