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math.FA2015
A short proof of the zero-two law for cosine functions
Jean Esterle
Let $(C(t))\in\mathbb{R}}$ be a cosine function in a unital Banach algebra. We give a simple proof of the fact that if lim sup$\_{t\to 0}\vert C(t)-1\_A\vert\textless{}2,$ then $li…
math.FA2015
A zero-sqrt(5)/ 2 law for cosine families
Jean Esterle
Let and let be the largest constant such that $sup\vert cos(na)-cos(nb)\vert \textless{} k(a)$ for implies that We show that if a…