Pro-nilfactors of the space of arithmetic progressions in topological dynamical systems

For a topological dynamical system $(X, T)$, $linmathbb{N}$ and $xin X$, let $N_l(X)$ and $L_x^l(X)$ be the orbit closures of the diagonal point $(x,x,ldots,x)$ ($l $ times) under the actions $mathcal{G}_{l}$ and $tau_l $ respectively, where $mathcal{G}_{l}$ is generated by $Ttimes Ttimes ldots times T$ ($l $ times) and $tau_l=Ttimes T^2times ldots times T^l$. In this paper, we show that for a minimal system $(X,T)$ and $lin mathbb{N}$, the maximal $d$-step pro-nilfactor of $(N_l(X),mathcal{G}_{l})$ is $(N_l(X_d),mathcal{G}_{l})$, where $pi_d:Xto X/mathbf{RP}^{[d]}=X_d,din mathbb{N}$ is the factor map and $mathbf{RP}^{[d]}$ is the regionally proximal relation of order $d$. Meanwhile, when $(X,T)$ is a minimal nilsystem, we also calculate the pro-nilfactors of $(L_x^l(X),tau_l)$ for almost every $x$ w.r.t. the Haar measure. In particular, there exists a minimal $2$-step nilsystem $(Y,T)$ and a countable set $Omegasubset Y$ such that for $yin Ybackslash Omega$ the maximal equicontinuous factor of $(L_y^2(Y),tau_2)$ is not $(L_{pi_1(y)}^2(Y_{1}),tau_2)$.