The interval of convergence for a power series is found by applying the ratio test. For \(f(x)\), we have:
\[
\lim_{{n \to \infty}} \left| \frac{a_{n+1}}{a_n} \right| = \lim_{{n \to \infty}} \left| \frac{x^{2(n+1)+1}/(2(n+1)+1)!}{x^{2n+1}/(2n+1)!} \right| =
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