Delaunay classification of positive singular solutions to the critical Lane--Emden system
We classify positive classical solutions of the Lane--Emden system \begin{align*} \begin{cases} -Δu=v^p & ~~\text{in }\mathbb{R}^n\setminus\{0\},\\ -Δv=u^q &~~ \text{in }\mathbb{R}^n\setminus\{0\}, \end{cases} \end{align*} with a nonremovable singularity at $0$ (i.e., $\lim_{x\to0}u(x)=\lim_{x\to0}v(x)=+\infty$), where $n\ge3$ and $p,q>0$ satisfy \[ \frac1{p+1}+\frac1{q+1}=\frac{n-2}{n}. \] Without additional growth or integrability assumptions, we establish radial symmetry about the origin, strict radial decrease, and global two-sided power bounds for both components. Furthermore, every such solution can be written as \[ u(x)=|x|^{-α}U(\log|x|),\qquad v(x)=|x|^{-β}V(\log|x|), \] where $α=\frac{n}{q+1}$, $β=\frac{n}{p+1}$, and $(U,V)$ is either the positive equilibrium $(A,B)=((\fracαβ)^{\fracα{2}}, (\fracαβ)^{\fracβ{2}})$ or a positive nonconstant periodic solution of the associated Emden--Fowler system. These periodic solutions form a Delaunay family: for every $\varepsilon\in(0,A)$, there is a unique profile pair, up to translation in $t=\log|x|$, with $\min_{\mathbb{R}}U=\varepsilon$. The Hamiltonian energy provides an alternative parameter, decreasing continuously and strictly from zero to the equilibrium energy as $\varepsilon$ increases from $0$ to $A$. The common least period of the two components tends to infinity as $\varepsilon\downarrow0$ and to the period of the linearized oscillation at $(A,B)$ as $\varepsilon\uparrow A$. The classification includes the case of a sublinear exponent.