It is clear that the minimal polynomial P of an algebraic number α over
Q(i) divides its minimal polynomial P0 ∈ Z[z] over Q. Moreover, for the leading
coefficient c0 ∈ Z[i] of the quotient P0/P, we have |c0| ≥ 1. Hence (P) ≤
(P0)/|c0| ≤ (P0). Also, by the same arguments as in [3] and [18] (see, e.g.,
Proposition 2 (ii) in [3]), we find that