Weak Solutions of Linear Differential Equations in Hilbert Spaces

We address the well-posedness of weak solutions for a general linear evolution problem on a separable Hilbert space. For this classical problem there is a well-known challenge of obtaining a priori estimates, as a constructed weak solution may not be regular enough to be utilized as a test function. This issue presents an obstacle for obtaining uniqueness and continuous dependence of solutions.
When formal energy estimates ara available, we provide a general notion of weak solution and, through a straightforward observation, obtain that arbitrary weak solutions have additional time regularity and obey an a priori estimate. This yields weak well-posedness. Our result rests upon a central hypothesis asserting the existence of a "good" Galerkin basis for the construction of a weak solution. A posteriori, a strongly continuous semigroup may be obtained for weak solutions, and by uniqueness, weak and semigroup solutions are equivalent.