Spectral extremal graphs for even factors
An even factor of a graph $G$ is a spanning subgraph in which every vertex has positive even degree. It is known that the minimum degree $\delta(G)\ge 2$ is a trivial necessary condition for $G$ to have an even factor. Recent spectral results for the existence of even factors used the certain complete-join graphs as exceptional extremal graphs. However, these graphs already contain $2$-factors and therefore are not genuine obstructions. This observation leads to the natural problem of determining the true sharp spectral threshold when the minimum degree is given. In this paper, we provide tight adjacency spectral radius conditions for a connected graph to contain an even factor, and characterize all extremal graphs, respectively. We also study the stronger requirement of a connected even factor, equivalently a spanning connected Eulerian subgraph. For this property, we also establish the corresponding sharp adjacency spectral radius condition and determine the unique extremal graph.