On the Hitchin-Thorpe inequality for gradient Ricci 4-solitons
Amir Babak Aazami
Abstract
We show that if an oriented closed 4-manifold $M$ admits a Ricci soliton metric, then its Euler characteristic and signature must satisfy $$χ(M) \geq \frac{3}{2}|τ(M)| - \frac{1}{16π^2}\!\int_M |\mathring{\text{Ric}}|^2,$$ where $\mathring{\text{Ric}}$ is the traceless Ricci tensor of the metric.
