Poincar\'e series of degree-two Chow varieties

Abstract

Let $C_{p, 2}(\mathbb{P}^{n})$ denote the Chow variety parametrizing effective algebraic $p$-cycles of degree two on $\mathbb{P}^{n}$. We show that its rational cohomology is pure Tate and concentrated in even degrees. An anti-invariant Chern class identity yields a presentation of the cohomology ring as a fiber product, which in turn determines the cohomology over $\mathbb{Z}[1/2]$. We identify the ordinary Poincar\'e polynomial with the known virtual formula, determine the optimal ranges over which the coefficients stabilize, and compute the nonrational Poincar\'e series of the stable space $D(2)$. For the Lawson--Michelsohn map $D(2)\longrightarrow D(\infty)$, the first nonzero rational homotopy kernel occurs in degree eight and is one-dimensional, while the integral kernel has rank one. Finally, we compute the relative homology of the pair $(D(2),BU)$ over $\mathbb{Z}[1/2]$ and show that additional cohomology generators and nonzero rational homotopy kernels arise in unbounded degrees.