This is Theorem 1, p. 530 in paper [1] by Pheidas.

**Reference**

[1] Thanases Pheidas, "[An undecidability result for power series rings of positive characteristics. II.](https://www.ams.org/journals/proc/1987-100-03/S0002-9939-1987-0891158-2/S0002-9939-1987-0891158-2.pdf)" Proceedings of the American Mathematical Society 100, 526-530 (1987), [MR891158](https://mathscinet.ams.org/mathscinet-getitem?mr=891158), [Zbl 0664.03008](https://zbmath.org/0664.03008).