A new characterization of Auslander algebras

Let $\Lambda$ be a finite dimensional Auslander algebra. For a $\Lambda$-module $M$, we prove that the projective dimension of $M$ is at most one if and only if the projective dimension of its socle soc\,$M$ is at most one. As an application, we give a new characterization of Auslander algebra $\Lambda$, and prove that a finite dimensional algebra $\Lambda$ is an Auslander algebra provided its global dimension gl.d\,$\Lambda\leq2$ and an injective $\Lambda$-module is projective if and only if the projective dimension of its socle is at most one.