On the location of roots of non-reciprocal integer polynomials

摘要

Let P be a polynomial of degree d with integer coefficients such that P(0) ≠ 0. Assuming that P has no reciprocal factors we obtain a lower bound on the modulus of the smallest root of P in terms of its degree d, its Mahler measure M(P) and the number of roots of P lying outside the unit circle, say, k. We derive from this that all d roots of P must lie in the annulus R _0 < |z| < R _1, where R _0 = R _0(d, k, M(P)) and R _1 = R _1(d, k, M(P)) are given explicitly. As an application, for non-reciprocal conjugate algebraic numbers α, α ′ of degree d ≥ 2 and of Mahler's measure M(α), we prove the inequality |αα′-1| > (12M(α)~2 log M(α)) ~(-d) Some lower bounds on the moduli of the conjugates of a Pisot number are also given. In particular, it is shown that if α is a cubic Pisot number, then the disc |z| ≤ α -1 + 0.1999α -2 contains no conjugates of α. Here the constant 0.1999 cannot be replaced by the constant 0.2. We also show that if α is a Pisot number of degree at least 4 and α′ is its conjugate, then |α α ′ - 1| > (19α ~2)~(-1).