Title: Generic reduction theory for Fermi sea topology in metallic systems

Abstract: Fermi sea in a metal can host exotic quantum topology, which determines its conductance quantization and is characterized by Euler characteristic $\chi_F$. Unlike gapped band topology described by the global feature of wave function, this topology of gapless system is associated with the geometry of Fermi sea, and thus probing and identifying $\chi_F$ are inherently difficult in higher-dimensional systems. Here, we propose a dimensional reduction theory for Fermi sea topology in $d$-dimensional metallic systems, showing that $\chi_F$ can be determined by the feature of so-called reduced critical points on Fermi surfaces, with theoretical simplicity and observational intuitiveness. We also reveal a nontrivial correspondence between the Fermi sea topology and the gapped band topology by using an ingenious mapping, of which $\chi_F$ exactly equals to the topological invariant of gapped topological phases. This provides a potential way to capture $\chi_F$ through the topological superconductors. Our work opens an avenue to characterize and detect the Fermi sea topology using low-dimensional momentum information.