# Asymptotic of integral $\int_{1}^{e^n}(1-\frac{\ln(x)}{n})^n\,dx$

How could we find the large-$$n$$ asymptotic of $$\int_{1}^{e^n}\left(1-\frac{\ln x}{n}\right)^n\,dx.$$ I have a suspicion that this is $$\sqrt{n}$$.

Denote $$t=\ln(x)/n$$, then $$t$$ varies from 0 to 1 and the the integral reads as $$n\int_0^1 ((1-t) e^{t})^ndt.$$ We have $$(1-t)e^t=(1-t)(1+t+t^2/2+\ldots)=1-t^2/2+O(t^3)=\exp(-t^2/2+O(t^3))$$ for small $$t$$ and $$(1-t)e^t<1$$ for $$0. Thus by Laplace method the asymptotics is the same as for the integral $$n\int_0^1e^{-nt^2/2}dt=\sqrt{n}\int_0^{\sqrt{n}}e^{-s^2/2}ds\sim \sqrt{\frac{\pi n}2}.$$