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? 



fixed point and homotopy fixed points
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.


