Real-rootedness of the Poincare polynomials of overline{mathcal M}_{0,n}: an AI-assisted proof

arXiv:2605.29151v1 Announce Type: cross Abstract: We prove real-rootedness for the Poincare polynomial [ P_n(t)=sum_{i=0}^{n-3} im H^{2i}(overline{mathcal M}{0,n};Q)t^i ] of the Deligne--Mumford moduli space overline{mathcal M}{0,n} of stable n-pointed rational curves, proving a conjecture of Aluffi--Chen--Marcolli. The proof starts from the Keel--Manin--Getzler recurrence, but its main new idea is a bivariate deformation F_m(y,t) of the Poincare polynomial. This deformation reveals a hidden interlacing structure not visible in the one-variable recurrence. For fixed t<0, the zero set of F_m in the y-direction is controlled by a Sturm--Rolle argument on the interval 0<y<1-t. The original polynomial is recovered on the slice y=1, and the ordered crossings of the moving roots through this slice give both real-rootedness and strict interlacing. Consequently, the Betti numbers of overline{mathcal M}{0,n} form an ultra-log-concave sequence. We further prove real-rootedness and ultra-log-concavity for the Poincare polynomial of the Fulton--MacPherson space P^1[n] of n ordered points in degenerations of the complex projective line. The proof for overline{mathcal M}{0,n} was obtained through an iterative AI-assisted workflow with Co-Mathematician, an agentic frontier-model system developed by Google DeepMind. The human role was to pose the problem, evaluate successive attempts, request repairs of gaps, compare the evolving argument with the literature, and assemble the final human-verifiable proof. Our additional human contribution was to observe that a similar residual deformation strategy applies to the Fulton--MacPherson spaces mathbb P^1[n], yielding the corresponding real-rootedness theorem.

Source: arXiv cs.AI | 2026-05-29