There are no two non-real conjugates of a Pisot number with the same imaginary part

Abstract

We show that the number $\alpha=(1+\sqrt{3+2\sqrt{5}})/2$ with minimal polynomial $x^4-2x^3+x-1$ is the only Pisot number whose four distinct conjugates $\alpha_1,\alpha_2,\alpha_3,\alpha_4$ satisfy the additive relation $\alpha_1+\alpha_2=\alpha_3+\alpha_4$. This implies that there exists no two non-real conjugates of a Pisot number with the same imaginary part and also that at most two conjugates of a Pisot number can have the same real part. On the other hand, we prove that similar four term equations $\alpha_1 = \alpha_2 + \alpha_3+\alpha_4$ or $\alpha_1 + \alpha_2 + \alpha_3 + \alpha_4 =0$ cannot be solved in conjugates of a Pisot number $\alpha$. We also show that the roots of the Siegel's polynomial $x^3-x-1$ are the only solutions to the three term equation $\alpha_1+\alpha_2+\alpha_3=0$ in conjugates of a Pisot number. Finally, we prove that there exists no Pisot number whose conjugates satisfy the relation $\alpha_1=\alpha_2+\alpha_3$.