A factorial polynomial $\phi(x)$ of degree $n$ has a unique representation $$\phi(x)=a_nx^{\underline{n}}+a_{n-1}x^{\underline{n-1}}+\ldots+a_1x^{\underline{1}}+a_0,\quad a_n\neq 0,$$ that is, if $\phi(x)$ has some other representation $$\begin{align}\phi(x)&=b_{n+k}x^{\underline{n+k}}+b_{n+k-1}x^{\underline{n+k-1}}+\ldots+b_{n+1}x^{\underline{n+1}}+\nonumber\\ &\quad+b_nx^{\underline{n}}+b_{n-1}x^{\underline{n-1}}+\ldots+b_1x^{\underline{1}}+b_0\nonumber\end{align},$$ then $$b_{n+k}=b_{n+k-1}=\cdots=b_{n+1}=0$$ and $$b_r=a_r,\quad r=0,1,\ldots,n.$$
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