"An Improved Estimator For Black-Scholes-Merton Implied Volatility" by Hallerbach (2004) (link to article) provides an equation (Eq. 24, Page 13, and below) for the implied volatility of a call option. What would be the equation for a put option? Thank you!
$$\sigma\sqrt{T} = \frac{\sqrt{2\pi}}{2(S+X)}\Biggl[2C+X-S+\sqrt{(2C+X-S)^2-1.85\frac{(S+X)(X-S)^2}{\pi\sqrt{XS}}}\Biggl]$$