Asymptotic Expansions of the Lognormal Implied Volatility : A Model Free   Approach
  We invert the Black-Scholes formula. We consider the cases low strike, large strike, short maturity and large maturity. We give explicitly the first 5 terms of the expansions. A method to compute all the terms by induction is also given. At the money, we have a closed form formula for implied lognormal volatility in terms of a power series in call price. 
Cyril Grunspan
q-fin.PR