It is known that two consecutive coefficients of a ternary cyclotomic polynomial $Φ_{pqr}(x)= ∑_k a_{pqr}(k)x^k$ differ by at most one. We characterize all k such that $|a_{pqr}(k)-a_{pqr}(k-1)|=1$. We use this to prove that the number of nonzero coefficients of the nth ternary cyclotomic polynomial is greater than $n^{1/3}$.
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We prove that for every ε > 0 and every nonnegative integer w there exist primes $p_1,...,p_w$ such that for $n = p_1... p_w$ the height of the cyclotomic polynomial $Φ_n$ is at least $(1-ε) c_w M_n$, where $M_n = ∏_{i=1}^{w-2} p_i^{2^{w-1-i}-1}$ and $c_w$ is a constant depending only on w; furthermore $lim_{w→∞} c_w^{2^{-w}} ≈ 0.71$. In our construction we can have $p_i > h(p_1... p_{i-1})$ for all i = 1,...,w and any function h: ℝ₊ → ℝ₊.
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