A unified family of counterexamples to Batyrev's non-negativity conjecture on stringy Hodge numbers

Abstract

Recently, Huang and Satriano determined the sharp dimension threshold for Batyrev's non-negativity conjecture on stringy Hodge numbers: the conjecture holds in dimensions at most four and fails in every dimension at least five. We show that their counterexamples fit into a single broad three-parameter family $Y_{m,d}:=V(\sum_{i=0}^m s_i g_i(x)+h(x))\subset\mathbb{P}^{m+d+1}$, where each $g_i$ is homogeneous of degree $d-1$ and $h$ is homogeneous of degree $d$, whose singular locus is a linear space $\Lambda\cong\mathbb{P}^m$. Blowing up $\Lambda$ gives a log resolution with one smooth exceptional divisor of discrepancy one. We compute the Hodge-Deligne polynomials of both the resolution and the exceptional divisor from their projective-space fibrations over $\mathbb{P}^d$, derive a master formula for $E_{\mathrm{st}}(Y_{m,d}\times(\mathbb{P}^1)^n)$, and prove the structure formula $h_{\mathrm{st}}^{p,q}(Y_{m,d}\times(\mathbb{P}^1)^n)=A_{p,q}+B_{p,q}-C_{p,q}$, where the three terms are non-negative. Thus the only possible source of negativity is $-C_{p,q}$, which is governed by the primitive middle cohomology of $Z_{m,d}=V(g_0,\ldots,g_m)\subset\mathbb{P}^d$. Off the diagonal, every negative entry equals $-C_{p,q}$, a binomially weighted sum of shifted primitive Hodge numbers of $Z_{m,d}$. On the diagonal, the behaviour is governed by the parity of $m+d$. We also obtain a complete classification for $0\le m\le d-1$, $d\ge3$, and $n\ge1$. The only non-counterexamples are $(m,d)=(0,3)$ for every $n\ge1$, and $(m,d)=(2,3)$ for $n\ge4$.