Degenerate C-distribution cosine functions and degenerate C-ultradistribution cosine functions in locally convex spaces

The main purpose of this paper is to investigate degenerate $C$-(ultra)distribution cosine functions in the setting of barreled sequentially complete locally convex spaces. In our approach, the infinitesimal generator of a degenerate $C$-(ultra)distribution cosine function is a multivalued linear operator and the regularizing operator $C$ is not necessarily injective. We provide a few important theoretical novelties, considering also exponential subclasses of degenerate $C$-(ultra)distribution cosine functions.