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The capacity of additive Gaussian noise (AGN) channels, $Y_t=X_t+V_t, t=1,\ldots, n$, $\frac{1}{n} {\bf E}\big\{\sum_{t=1}^n |X_t|^2 \big\}\leq \kappa,\kappa \in [0,\infty)$, with time-invariant channel input feedback strategies,is characterized and conditions are identified for entropy rates, and limit ofaverage power to exist, when the noise is described by {\it stable andunstable} autoregressive models, AR$(c)$, $V_t=cV_{t-1}+ W_t, V_0=v_0, t=1,\ldots, n$, where $c\in (-\infty,\infty)$, $W_t, t=1,\ldots, n$, is a zeromean, variance $K_W$, independent Gaussian sequence, independent of $V_0$. Forstable AR$(c), c\in (-1,1)$ the conditions are necessary and sufficient forasymptotic stationarity of the processes $(X_t, Y_t), t=1, 2, \ldots$. Newclosed form capacity formulas and lower bounds are derived, for the AR$(c),c\in (-\infty,\infty)$ noise, which are fundamentally different from existingformulas in the literature, and illustrate multiple regimes of capacity, as afunction of the parameters $(c,K_W,\kappa)$, as follows.\\ 1) feedbackincreases capacity for the regime, $c^2 \in (1, \infty),$ for $\kappa >\frac{K_W\big(1+\sqrt{4c^2-3}\big)}{2\big(c^2-1\big)^2}$, \\ 2) feedback doesnot increase capacity for the regime $c^2 \in (1, \infty)$, for $\kappa \leq\frac{K_W\big(1+\sqrt{4c^2-3}\big)}{2\big(c^2-1\big)^2}$, and \\ 3) feedbackdoes not increase capacity for the regime $c \in [-1,1]$, for $ \kappa \in[0,\infty)$.

Time-Invariant Feedback Strategies Do Not Increase Capacity of AGN Channels Driven by Stable and Certain Unstable Autoregressive Noise