  In the Black-Scholes model, the absence of arbitrages imposes necessary
constraints on the slope of the implied variance in terms of log-moneyness,
asymptotically for large log-moneyness. The constraints are used for example in
the SVI implied volatility parameterization to ensure the resulting smile has
no arbitrages. This note shows that those no-arbitrage contraints are very
mild, and that arbitrage is almost always guaranteed in a large range of slopes
where the contraints are enforced.
