I'm studying on these notes, my question in about a proof on page 63. Basically this is my question:
Suppose $R$ local noetherian of positive depth, $M$ a module of finite gorenstein dimension $n\geq1$ and of depth 0. Take an exact sequence $0\rightarrow N\rightarrow P\rightarrow M\rightarrow0$ where $P$ is totally reflexive (we can take it projective) and the Gorenstein dimension of $N$ is $n-1$. The notes claim that $\mathrm{depth}\;N=1$, why this is true? (it would be ok also if you could tell me why $\mathrm{depth}\;N\geq1$).