My book has considered the effects of supply and demand changes on price (specifically the equilibrium price and quantity themselves) when the supply and demand themselves have constant elasticity and then go on to claim this statement
When the effect considered is local, and locally, the elasticity is approximately constant if the demand is "smooth".
As a mathematician and a beginner in micro-economics, I fail to understand this statement from a rigorous viewpoint. How do you prove this mathematically?