  It is known that two consecutive coefficients of a ternary cyclotomic
polynomial $\Phi_{pqr}(x)=\sum_k a_{pqr}(k)x^k$ differ by at most one. In this
paper we give a criterion on $k$ to satisfy $|a_{pqr}(k)-a_{pqr}(k-1)|=1$. We
use this to prove that the number of nonzero coefficients of the $n$th ternary
cyclotomic polynomial is greater than $n^{1/3}$.
