can someone help explain the t interval in this series

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Why do I often find the interval t in Euler's polynomial is often different, some say $|t|<π$ & some say $|t|<2π$. Why is that, at first I thought |t| this has no effect in the series since this Euler series departs from $\frac{1}{cos(t)}$. Can anyone explain this?

https://www.google.com/url?sa=t&source=web&rct=j&url=https://ijpam.eu/contents/2012-78-1/5/5.pdf&ved=2ahUKEwirlYjO-KLyAhVfILcAHRIDBdoQFnoECAMQAg&usg=AOvVaw1I4JMEZmXWbxyYUNYSSyPv