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Let $G$ be a group and $X$ be a $G$-space (finite G-CW-complexe when needed).

Let $p$ a prime number and $G= \mathbf{Z}/p\mathbf{Z}$, If I'm not wrong Miller-Lannes,... theory provides tools and criteria to compare and compute the space of fixed points and homotopy fixed points $X^{G}\rightarrow X^{hG}$ (after $p$-completion.)

Question Do we have a similar theory and conjecture (a la Sullivan) in the case where $G=\mathbf{Z}$ ? If yes, what should be the correct formulation?

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We certainly don't expect to get anything similar to the Sullivan conjecture. In the case where $G$ acts trivially, the Sullivan conjecture tells us that the space of maps from $B(\mathbb{Z}/p)$ to $X$ is homotopy equivalent to the space of constant maps, but that will not be the case if we replace $B(\mathbb{Z}/p)$ by the space $B(\mathbb{Z})=S^1$.

To explain how to compute what you do get, suppose that the generator of $\mathbb{Z}$ acts on $X$ as a homeomorphism $f:X\to X$. Using $\mathbb{R}$ as a model for $E\mathbb{Z}$, it is not hard to see that $X^{h\mathbb{Z}}$ is the homotopy pullback of the diagonal map $\delta:X\to X\times X$ and the map $\alpha:X\to X\times X$ given by $\alpha(x)=(x,f(x))$. At least if $X$ is simply connected and we take cohomology with coefficients in a field, this gives an Eilenberg-Moore spectral sequence $$ \text{Tor}^{**}_{H^*(X)\otimes H^*(X)}(H^*(X),H^*(X)) \Longrightarrow H^*(X^{h\mathbb{Z}}). $$ Here the two copies of $H^*(X)$ in brackets are regarded as modules over $H^*(X)\otimes H^*(X)=H^*(X\times X)$ using $\delta^*$ and $\alpha^*$ respectively. I think that this is probably a practical method of calculation in some simple cases.

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