Abelian spectral topology of multifold exceptional points

The advent of non-Hermitian physics has enriched the plethora of topological phases to include phenomena without Hermitian counterparts. Despite being among the most well-studied uniquely non-Hermitian features, the topological properties of multifold exceptional points, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>n</a:mi></a:math>-fold spectral degeneracies (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>EP</b:mi><b:mi>n</b:mi><b:mi mathvariant="normal">s</b:mi></b:mrow></b:math>) at which also the corresponding eigenvectors coalesce, were only recently revealed in terms of topological resultant winding numbers and concomitant Abelian doubling theorems. Nevertheless, a more mathematically fundamental description of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>EP</d:mi><d:mi>n</d:mi><d:mi mathvariant="normal">s</d:mi></d:mrow></d:math> and their topological nature has remained an open question. To fill this void, in this article, we revisit the topological classification of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>EP</f:mi><f:mi>n</f:mi><f:mi mathvariant="normal">s</f:mi></f:mrow></f:math> in generic systems and systems with local symmetries, generalize it in terms of more mathematically tractable (local) similarity relations, and extend it to include all such similarities as well as nonlocal symmetries. Through the resultant vector, whose components are given in terms of the resultants between the corresponding characteristic polynomial and its derivatives, the topological nature of the resultant winding number is understood in several ways: in terms of (1) the tenfold classification of (Hermitian) topological matter, (2) the framework of Mayer-Vietoris sequence, and (3) the classification of vector bundles. The classification scheme further predicts the existence of topological bulk Fermi arcs protected by a <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi mathvariant="double-struck">Z</h:mi><h:mn>2</h:mn></h:msub></h:math>-invariant, induced by nonlocal symmetries, dubbed <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi mathvariant="double-struck">Z</j:mi><j:mn>2</j:mn></j:msub></j:math>-protected Fermi arcs. Our work reveals the mathematical foundations on which the topological nature of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:mi>EP</l:mi><l:mi>n</l:mi><l:mi mathvariant="normal">s</l:mi></l:mrow></l:math> resides, enriches the theoretical understanding of non-Hermitian spectral features, and will therefore find great use in modern experiments within both classical and quantum physics.