A generalization of the Shafer-Fink inequality

Abstract

In this article we show a tecnique based on the Weierstrass product for the sine and cosine function and the bisection formula for the cotangent function that leads to a generalization of the classical Shafer-Fink inequality $ \frac{3 x}{1+2\sqrt{1+x^2}} < \arctan x < \frac{\pi x}{1+2\sqrt{1+x^2}}$. Other algebraic approximations are also shown, including one that follows from the Chebyshev expansion for the arctangent function in the interval $[-1,1]$.