Unconventional models and Peano Arithmetic

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I'm trying to show that $\mathbb{Z}[x]^+ \models \mathsf{PA}^-$. What are the initial segments of this model?

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To show that a certain model satisfies $PA^{-}$, make a list of all the axioms of $PA^{-}$ (they are the 15 axioms listed on the Wikipedia article). Then check, one at a time, that $\mathbb{Z}[x]^+$ satisfies each of these axioms.